Glossary of Rock & Mineral Terms

What some of the techincal & specialized words mean.
Don't worry if it feels overwhelming.
you don't need to know the exact words to appreciate rocks & minerals.
It only helps when you want to describe them more accurately.

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  acicular / radiating needles ~ crystals that grow in fine needles
  adamantine ~ very shiny like a gemmy crystal,  almost brilliant
  agate ~ a waxy variety of cryptocrystalline quartz (chalcedony) in which the colours are in bands, clouds or distinct groups
  agate nodule ~ a lense-shaped agate nodule
  • fill holes can be seen & affect pattern of banding
  • rapid deposition of groundwater causes dense agate to form
  alloy ~ a combination of metals to make a new metal compund
  Alluvial Deposits Gem deposits found in water after they have been separated from the mother rock. (GS)
  amygdaloid ~

volcanic rocks that contain numerous gas cavities filled with secondary minerals such as zeolites, calcite, chalcedony or quartz

  • filled cavities are called amygdules
  aphanitic ~ pertaining to a texture of rocks in which the crystalline constituents are too small to be distinguished with unaided eye
  • used to describe igneous rock rhyolite
  Asterism The star effect that you see in star sapphires or rubies, for example. This is usually caused by tiny silk rutile inclusions in the stone. The effect can be four- or six- rayed. (GS)
  book ~ a stack of thin, flat crystals that form a "book", e.g. book of mica
  Baroque Brilliant Cut A round shaped stone that has a minimum of fifty-eight facets. (GS)
  botryoidal ~ shaped like a bunch of grapes, e.g. botryoidal chalcedony
  • chalcedony forms bubbly surface
  • found in the geodes of Kentucky & Indiana
  breccia ~ means angular, refers to a type of rock where angular pieces of rocks are “cemented” together with fine grained sand, type of sedimentary rock
  button ~ a small cluster or plate of crystals, e.g. amethyst button
  Cabochon Cut A gem that is cut round without facets into the shape of a smooth polished dome. It lacks the facets that are on most stones. (GS)
  Carat A unit of weight for gems. A carat is one fifth of a gram (0.2g). (GS)
  chalcedony ~ ryptocrystalline quartz and much chert
  • material agate is made of
  • has a great array of colours: blue, gray, black, off-white, purple (deccomposition of iron-bearing minerals)
  • in Hall’s Gap geodes
  • sulfide mineralization occurs in chalcedony geode
  Chatoyancy This is the "cat's eye" effect that is seen in chrysoberyl, for example, here an eye can be seen when the stone is moved under a strong light. This is caused by the narrow inclusions in the stone. (GS)
  chatoyant ~ shines like a cat’s eye because of fibers
  chemicals ~ everything on earth is made up of the 103 known chemical elements, including rocks, mineral,
air, water, plants & animals
  chert ~ a cryptocrystalline variety of quartz
  • occurs as nodules, lenses or layers in limestone and shales
  Clarity Referring to a stone's lack of inclusions or other visual defects. (GS)
  Cleavage The plane of weakness of some gems where they will split apart with smooth surfaces. Gems with perfect cleavage are likely to break when being cut or faceted. (GS)
  cleavage ~ the property to break along smooth lines or planes, the mineral has a shape it wants to be &
breaks along those lines to keep that shape
  Color Used in the evaluation of a gem. The quality of a gem can based on either the presence or the absence of color. (GS)
  common names ~ when rocks and minerals are known by names that are descriptive, not scientific
  conchoidal ~ curved break like what happens with thick glass or bottle bottom, shell shaped, can be rough or smooth
  concretion ~ spherical mass that is separate from the rock around it, usually weathers out of host rock, grows from the inside out
  • a nodular or irregular concentration of certain authigenic constituents of sedimentary rocks and tuffs
  • developed by the localized deposition of material from solution, generally about a central nucleus
  • are solid, grow from the center outward & are generally noncrstalline though some crystals have been observed
  • concretions are formed by the deposition of distinct minerals, different from the surrounding rock, very firmly cemented around a nucleus
  • most common cementing materials are calcite, siderite & silica
  • parts of plants, animals & well-preserved fossils may be found at the nucleus
  • harder than enclosing rock
  Crown The top of a gemstone above the girdle. (GS)
  cryptocrystalline ~ when the crystals are so small, they cannot be seen except with a microscope
  crystal habit ~ the form that the crystals of a mineral take when they have enough space and time to grow properly

crystal shape ~ the form or habit of a mineral, the shape that the mineral takes if it has the time & space
to grow properly

  crystals ~ minerals that form slowly have a distinctive crystal shape

cubic ~ 6 equal, square faces

  Culet The lowest part of a gemstone. This looks the tip or point of the stone. (GS)
  dendritic ~ tree-like, branching, tree-like growths
  Density The ratio of a gemstone when compared to the weight of an equal volume of water. This means how heavy a gemstone is compared to the same volume of water. Also known as "specific gravity" for solids. (GS)
  Dichroism A term meaning the ability of some gems to display a second shade of the same color when viewed from a different angle. A dichroscope can see this change, and is used for identifying certain stone. (GS)
  dimension stone ~ stones that are used for construction or architectural features are often described as dimension stones. The Dimension Stone Industry defines rocks based on what the rocks look like and how hard they are (namely what they can be used as). The main "rock types" according to the dimension stone industry are "granite", marble and slate.
  Dispersion The property of a transparent stone to split light into the seven spectral colors, causing the "fire" which is refracted by the internal facets. Diamond has a very high dispersion, hence its high amount of fire. (GS)
  dog-tooth ~ shaped like the canine tooth, like a dog's tooth, eg dog-tooth calcite
  Double Refraction The ability of most gems to split rays of light into two rays. (GS)
  dull / earthy ~ very dull, mainly in minerals that are porous
  earth's Crust ~ the earth's crust is made of solid, hardened rocks & minerals
  enhydro ~ an agate with a visible bubble moving around inside liquid when tilted
  erosion ~ the process through which mountains are broken down into boulders & sand
  Facet The cut and polished flat plane of a gemstone. There can be dozens of facets on a stone. (GS)
  fibrous ~ used to describe mineral crystals that are long and thin and look like fibers
  Fire The rainbow or colors that light rays form as they move through a gemstone. This is another word for "dispersion". (GS)
  floaters ~ fully formed or doubly-terminated crystals that are unattached to other crystals
  • found in geode cavities & also in vugs such as "Herkimer Diamonds"
  Fluorescence The ability of some gems to appear a different color when viewed under ultraviolet light. If or not a stone has fluorescence is a valuable aid in gem identification. (GS)
  fossil ~ the remains of plants & animals that have been replaced by minerals
  fracture ~ is the way a mineral breaks when it won’t break on a cleavage plane
  Full Cut A round-shaped, brilliant-cut gemstone. (GS)
  gemmy ~ clear or transparent like a gem, used to describe transparency of a mineral
  gemstone ~ a precious or semi-precious stone that is used in jewellery, often clear or gemmy
  gemstones ~ rocks & minerals that have been cut & polished, used for decoration and are usually rare and
  geode ~ a round rock that has a hard "skin" on the outside and that usually is hollow on the inside, often lined or filled with mineral crystals, most common type is the quartz geodes
  • hollow, globular bodies
  • subspherical shape
  • clay film between geode wall & the enclosing limestone matrix which is like a skin or crust
  • an outer chalcedony layer
  • an interior drusy lining of inward projecting crystals
  • evidence of expansion or growth
  • slow deposition of groundwater causes crystals to grow
  • estimated it takes 240 million years to form
  • geodes form either in sedimentary rock or in basalts/andesites
  • hollows appeared in sedimentary rocks either because of animal burrows, tree roots or mud balls
  • word "geode" derived from Latin meaning "earthlike", rounded shape
  • found in USA: Montana, Indiana, Iowa, Illinois, Pennsylvania (Lancaster Co. goethite geodes), Tennessee (Loretto), Oklahoma (Comanche County etc.), Michigan (Houghton), Dugway geodes, Colorado (Chaffee County ,Marshall Pass), Hauser Geode Beds, California
  • amethyst geodes from Artigas, Uruguay or near Lajeado, Rio Grande Do Sul, Brazil



geode ~ a sphere with a hollow inside, often lined with crystals, grows from the outside in

  geologist ~ a scientist that studies rocks & minerals and the earth sciences
  Girdle The widest point in circumference of a gem. This is the point where a gem is usually held by fingers or tweezers for examination. (GS)
  glassy ~ shiny like glass, found in 70% of minerals, vitreous
  "granite" dimension stone ~ according to the Dimension Stone Industry this includes granite, gneiss, gabbro, anorthosite and some sedimentary rocks (Newfoundland). They are used for architectural sones, construction, flooring, cladding of buildings (exterior "skin"), counter tops, curbs.
  hardness ~ how easy it is to scratch a mineral
  Heat Treatment The application of high heat to a gemstone in order to improve its color and clarity. (GS)
  hexagonal prism with pyramid termination ~ hexagonal cross-section, with pointy ends
  host rock ~ this is the rock that surrounds a mineral specimen, the matrix
  igneous ~ rocks made from fire & heat, liquid magma that has cooled to form rocks
  Inclusions Foreign matter that is "included" within a stone. This may be a foreign body such as a crystal, a gas bubble or a pocket of liquid. There are many varieties of inclusions and they are important visual clues for identifying the type of gemstone and for identifying the origin of the stone. (GS)
  industrial geode ~ a.k.a. garden geodes - found in the garden, solid all the way through, milky quartz
  Iridescence Effect caused by the interference of light on thin films within the gemstone. (GS)
  ironstones ~ large concretions of siderite & calcite found in shales assoicated with coal beds in Kentucky
  • name not much in use now
  • collected just south of Holland, Michigan
  Irradiation Exposing gemstones to radioactive rays from x-rays or other material to change or enhance the original color. Blue topaz is always irradiated, for example. (GS)
  Keokuk Geode ~ a specific type of geode which can be found in a 45 mile radius of Keokuk, Iowa in the Tri-State area - Iowa, Illinois, Missouri
  • geode is Iowa’s official state rock
  • most geodes come from strata of the lower Warsaw Formation, a rock unit of Mississippian age
  • formed in sedimentary rock- shale, shaley dolomite & limestone
  • original concretions thought to made of limestone or anhydrite which then dissolved and was relaced by chalcedony for the skin and interiors filled in over long periods of time
  • source of mineralizing water is speculative as quartz is only weakly soluble
  • predominately quartz crystals
  • silicates: crystaline quartz, kaolinite
  • carbonates: calcite, aragonite, ferroan dolomite, malachite, smithsonite, stilpnosiderite
  • zinc sulfide
  • sulphides: iron pyrite, sphalerite, chalcopyrite, millerite
  • sulfates: barite, selenite, jarosite
  • oxides: goethite, hematite, pyrolusite
   lapidary ~ the craft of forming shapes, items, gemstones from rocks & minerals
  Lapidary The science and art of cutting and polishing gems to their finished state. (GS)
  Luster The outward appearance of a gem or organic material. The quantity and quality of light that is reflected from the surface of a stone. Luster is important especially when evaluating the quality of pearls. (GS)

luster ~ how shiny something is; words used to describe the way light reflects off of the surface of a specimen

  lythophysae ~ hollow, bubblelike structures composed of concentric shells of finely crystalline alkali feldspar, quartz and other minerals
  • found in certain silicic volcanic rocks such as rhyolite and obsidian
  • a.k.a. thunder egg in the USA
  • in France, come from the Esterel massif in the south-eastern part of France
  • found in Ploand
   "marble" dimension stone ~ according to the Dimension Stone Industry this includes travertine, alabaster, calcareous stones, virginite (Newfoundland). These are used for interior floors that get little traffic, facing stone ("skin"), monuments and sculptures.
  massive ~ not a crystal, a piece of a mineral with no particular shape
  massive ~ a mineral with no distinct crystal shape, large chunk of inter-grown minerals
  matrix ~ the rock that mineral specimens are found in are called the matrix or host rock

matrix ~ the host rock that a mineral specimen or crystal is found in or on, bedrock

  metallic ~ shiny like polished metal, highly reflective, usually opaque
  metamorphic ~ igneous or sedimentary rocks that have been changed through extreme heat &
pressure due to movement of the earth’s crust

mineral ~ non-living matter, chemically the same all the way through

  minerals ~ all rocks are made of one or more of the 3000 known minerals
  Mohs Hardness Scale Numerical scale ranging from 1 to 10 developed by Friedrich Mohs that assigns a rating to a gem according to its ability to resist scratching. The hardest is 10 (diamond) and the softest is 1 (talc). (GS)

no cleavage ~ does not break regularly

  nodule ~ rounded, spherical or kidney shaped rock or mineral, often with a "skin"; because it is harder than the surrounding rock it is in, it weathers out intact; e.g. pyrite nodule
  • small more or less rounded body generally somewhat harder than the enclosing sediment or rock matrix
  • is solid
  • agate, jasper & chalcedony form in nodules
  • other common minerals that occur in nodules are siderite, gypsum, calcite, quartz and barite/celestite
  ornamental stone ~ a rock or stone that is used for decoration, as an ornament - such as for lamps, book ends, wall and floor tile, tables, sculptures - usually because of its beautiful colour or pattern
  opaque ~ no light can pass through it, massive pieces of most minerals are opaque, almost all metallic minerals are opaque
  opaque ~ cannot see through it at all, blocks all light, casts a solid shadow, acts like a wall
  Opaque A term used for gemstones that you cannot see any light passing through the gem. Lapis and malachite are an example of this. (GS)
  paleontologist ~ a scientist who studies paleontology, learning about the forms of life that existed in former
geologic periods, chiefly by studying fossils
  Pavilion The lower portion of a gemstone that begins just below the girdle. (GS)
  pearly ~ like a pearl, play of colors on surface
  perfect cleavage ~ some minerals break easily into flat pieces and this is called perfect cleavage, cleavage means "to come apart"
  physical properties  ~ the common visible and tangible characteristics used in the identification & study
of minerals
  Pleochroism The ability of certain gems to display two or more colors when viewed from different angles. This is a term also used for Dichroism and trichroism. (GS)
  porphyry ~ a rock that is composed of angular pieces of rock "cemented" together - formed through the sedimentary process
  Portuguese cut A particular type of faceting where the stone is cut with two rows of rhomboidal and three rows of triangular facets above and below the girdle. (GS)
  prismatic ~ crystal shaped like a prism with flat sides and a pointy termination or end, e.g. quartz crystal
  Refraction The bending of light as it enters a medium and slows down. (GS)
  Refractive Index A process using a refractometer to measure the speed and angle of light entering a gemstone. Very important for gem identification. (GS)
  rhomb ~ looks like a squished box, short form for rhombohedral, resembling a prism with six four-sided faces
  rhyolite ~ the aphanitic equivalent of a granite
  • thunder eggs form in rhyolitic rocks - Oregon & southeastern California
  road cut ~ a road cut is where the rock has been cut to let the road go through, rather than making the road go up & down hills, rocks are cut to flatten the road
  rock cycle ~ rocks are constantly forming, wearing down and forming again, very slowly however
  rockhound ~ someone who collects rocks and minerals, a lover of rocks, minerals & fossils who collects specimens in the field
  rocks ~ non-living matter, made of 2 or more minerals
  Rough In gemology, this refers to the raw, natural state in which gems are found, before they are cut. (GS)
  Rutiles Needle-like inclusions (or foreign matter) within stones. These can produce some gem phenomena as an asterism (star) or cat's eye (chatoyancy.) (GS)
  schiller ~ when the light hits some minerals, they almost glow and show a bronzy or peacock color that changes with how the light hits the surface, a deep luster, iridescent
  schiller ~ colors shimmer or flash when the light hits the surface in a certain way
  sedimentary ~ layers of sand, clay & bits of rock laid down by water & turned to rock, often contains fossils
  sedimentary ~ rocks that are formed when layers of sand, clay & bits of rock are laid down by water & turned to rock, often contain fossils
  Septarian Nodule ~ septarian nodules that have a hollow cavity are sometimes known as "septarian nodule geodes" or "geodes"
  • Utah septarian nodules formed in ancient sea floor during the Cretaceous period, 50 - 70 million years ago - filled with calcite - yellow centres (calcite), brown lines (argonite), grey rock (limestone), white or clear (barite)
  septarium ~ a roughly spheroidal concretion, generally of limestone or clay-ironstone
  • cut into polyhedral blocks by radiating and intersecting cracks which have been filled (and the blocks cemented together) by veins of some material, generally calcite
  • some contain fossils
  • a.k.a. Septarian Nodule, Septarian Boulder, Turtle Stone
  • presence of an "Avenue of Entry"
  • in southern Illinois, they contain fluorite, sphalerite, witherite & sometimes galena
  • in Utah, the type has a different shape & may be filled with agate
  • thunder egg of Oregon is a septaria - cousin
  • most geodes coming from France are in fact septaria (Drome, Ardeche, Hautes-Alpes, Remuzat)
  Sheen This effect resembles luster, and is caused by light reflection from inclusions or texture inside the gem. Luster is light reflected from the surface of the gem and sheen is reflection from inside the gemstone. (GS)
  Single Cut Stones with seventeen facets or fewer. (GS)
  "slate" dimension stone ~ according to the Dimension Stone Industry (Newfoundland) it is used for roofing, flooring, counters, architectural construction & landscaping - depending on the quality.
  Species The term used to designate a family of gemstones. For example, corundum is a species that contains the varieties sapphire and ruby. The Quartz family contains amethyst, citrine, and chalcedony, to name a few. (GS)
  Specific Gravity (same as density) The ratio of a gemstone when compared to the weight of an equal volume of water. This means how heavy a gemstone is compared to the same volume of water. Also known as "specific gravity" for solids. (GS)
  specific gravity ~ how heavy something feels when compared to what you would expect,
heft, weight, mass, density
  specimen ~ a term used to refer to a mineral or rock sample, a piece one is studying or using as an example
  spherulite ~ a small radiating & usually concentrically arranged aggregation of one or more minerals generally of spherical or spheroidal shape
  • formed by the radial growth of acicular crystals in a rigid glass about a common centre or inclusion
  Step Cut A gem cut with rectangular facets along the perimeter.  (GS)
  striations ~ parallel lines on the face of a crystal, looks like regularly spaced ridges or scratches

sub-metallic ~ soft shine like dull metal

  Swiss Cut A gem cut consisting of thirty-three facets. (GS)
  Table The flat top part of a gemstone. The table is the largest facet. (GS)
  tabular ~ divide easily into thin sheets, a stack is know as a “book”
  termination ~ the end of a crystal that has grown properly, can be rounded, flat or pointed
  termination ~ the point at the end of a crystal
  thunderegg ~ geodelike body commonly containing opal, agate or chalcedony weathered out of welded tuff or lava
  • form only in rhyolitic rocks, almost always in association with perlite
  • exterior surface is "warty" & it has a rind
  • interior cross-section commonly exhibits a star-shaped outline
  • interiors are filled with chalcedony, agate or opal
  • when hollow, thunder eggs are sometimes referred to as geodes
  • no thunder eggs exist in the USA east of Colorado except for a single locality near Grand Marais on Minnesota’s North Shore
  • is the common name for lithophysae
  translucent ~ some light can pass through it but it is not clear; many minerals that are opaque can be made to appear translucent if a thin enough slice is cut off & light shone through it; e.g. wax paper
  Translucent A quality of a gemstone transmitting light imperfectly so that one cannot see through the stone clearly. Star sapphire is an example of this quality. (GS)
  translucent ~ see shadows and shapes through it when held up to the light, details not clear, is frosted or
cloudy, like looking through wax paper or light
  transparency ~ a physical characteristic of minerals, used to describe how much light can pass through a specimen

ransparency ~ describes if you can see through something or not

  transparent ~ clear, light can pass through, e.g. a piece of clear glass
  transparent ~ clear, see through clearly all the way, like a plain window glass or clear plastic wrap,
“gemmy” like a gemstone
  Treated stone A stone that has been heated, dyed, irradiated, or stained in order to improve the color or the clarity. Also pertains to gems that have their cracks or fractures concealed by filling the material. (GS)
  Trichroism A property of a stone that will show three colors or shades of the same color when the stone is viewed through a dichroscope. (GS)
  turtle stone ~ a septarium with distinctive external markings on the outside resembling the shell of a turtle
  uneven fracture ~ rough surface, not smooth
  vesicle ~ a small circular enclosed space
  • a small cavity in an aphanitic or glassy igneous rock, formed by the expansion of a bubble of gas or stem during the solidification of the rock
  vitreous ~ shiny like glass, found in 70% of minerals, glassy
  Vitreous Luster The most common gem luster. This is a luster with a shiny, glass-like appearance. (GS)
  vug ~ a hollow space in a rock where crystals often grow, e.g. Herkimer diamond in a vug
  • a cavity, often with a mineral lining of different composition from that of the surrounding rock
  • can form spherically, in veins, irregular openings
  • vugs do not generally weather out of the host rock
  waxy ~ looks softly shiny like wax, like the surface of a wax candle
  Zoning (color zoning) A term that describes the uneven distribution of color in a gemstone. Zoning is best seen when looking at the stone through the top table facet. (GS)

Gemstone Glossary (GS) (courtesy of GemSelect
sources: Dictionary of Geological Terms

For a Geological Dictionary go to


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