Glossary of Rock & Mineral Terms
|
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z |
| A | |
| 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
|
|
| 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
|
|
aphanitic ~ pertaining to a texture of rocks in which the crystalline
constituents are too small to be distinguished with unaided eye
|
|
| 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) | |
| B | |
| 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
|
|
| 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 | |
| C | |
| 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
|
|
| 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
|
|
| 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
|
|
| 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 |
|
| 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) | |
| D | |
| 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 | |
| E | |
| 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 | |
| F | |
| 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
|
|
| 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) | |
| G | |
| 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 valuable |
|
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
|
|
|
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. | |
| H | |
| 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 | |
| I | |
| 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
|
|
| 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) | |
| J | |
| K | |
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
|
|
| L | |
| 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
|
|
| M | |
| "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) | |
| N | |
|
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
|
|
| O | |
| 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) | |
| P | |
| 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 | |
| Q | |
| R | |
| 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
|
|
| 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) | |
| S | |
| 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"
|
|
septarium ~ a roughly spheroidal concretion, generally of limestone or
clay-ironstone
|
|
| 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
|
|
| 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) | |
| T | |
| 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
|
|
| 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 | |
| U | |
| uneven fracture ~ rough surface, not smooth | |
| V | |
vesicle ~ a small circular enclosed space
|
|
| 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
|
|
| W | |
| waxy ~ looks softly shiny like wax, like the surface of a wax candle | |
| X | |
| Y | |
| Z | |
| 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
http://www.gemselect.com:80/gem-info/glossary.php)
sources: Dictionary of Geological Terms
For a Geological Dictionary go to http://www.geologyshop.co.uk/dictio~1.htm
|
RocksForKids HOME Table of Contents Contact Us © 1999 - 2013 GMB Services Privacy Policy About Us About This Site CCFMS |