Glossary of Rock & Mineral Terms
RocksForKids.com
Click here for Gemstone Glossary
- alloy ~ a combination of metals to make a new metal compund
- book ~ a stack of thin, flat crystals that form a "book", e.g. book of mica
- 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
- common names ~ when rocks and minerals are known by names that are descriptive, not scientific
- 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
- dendritic ~ tree-like
- fibrous ~ used to describe mineral crystals that are long and thin and look like fibers
- 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
- 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
- host rock ~ this is the rock that surrounds a mineral specimen, the matrix
- lapidary ~ the craft of forming shapes, items, gemstones from rocks & minerals
- massive ~ not a crystal, a piece of a mineral with no particular shape
- matrix ~ the rock that mineral specimens are found in are called the matrix or host rock
- 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
- 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
- perfect cleavage ~ some minerals break easily into flat pieces and this is called perfect cleavage, cleavage means "to come apart"
- prismatic ~ crystal shaped like a prism with flat sides and a pointy termination or end, e.g. quartz crystal
- rhomb ~ looks like a squished box, short form for rhombohedral, resembling a prism with six four-sided faces
- road cut ~ a road cut is where the rock has been cut to let the road go through
- rockhound ~ someone who collects rocks and minerals
- 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
- sedimentary ~ rocks that are formed when layers of sand, clay & bits of rock are laid down by water & turned to rock, often contain fossils
- specimen ~ a term used to refer to a mineral or rock sample, a piece one is studying or using as an example
- striations ~ parallel lines on the face of a crystal
- termination ~ the end of a crystal that has grown properly, can be rounded, flat or pointed
- 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
- transparency ~ a physical characteristic of minerals, used to describe how much light can pass through a specimen
- transparent ~ clear, light can pass through, e.g. a piece of clear glass
- vitreous ~ shiny like glass
- vug ~ a hollow space in a rock where crystals often grow, e.g. Herkimer diamond in a vug
Gemstone Glossary (courtesy of GemSelect http://www.gemselect.com:80/gem-info/glossary.php)
- Alluvial Deposits Gem deposits found in water after they have been separated from the mother rock.
- 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.
- Baroque Brilliant Cut A round shaped stone that has a minimum of fifty-eight facets.
- 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.
- Carat A unit of weight for gems. A carat is one fifth of a gram (0.2g).
- 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.
- Clarity Referring to a stone's lack of inclusions or other visual defects.
- 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.
- Color Used in the evaluation of a gem. The quality of a gem can based on either the presence or the absence of color.
- Crown The top of a gemstone above the girdle.
- Culet The lowest part of a gemstone. This looks the tip or point of the stone.
- 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.
- 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.
- 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.
- Double Refraction The ability of most gems to split rays of light into two rays.
- Facet The cut and polished flat plane of a gemstone. There can be dozens of facets on a stone.
- Fire The rainbow or colors that light rays form as they move through a gemstone. This is another word for "dispersion".
- 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.
- Full Cut A round-shaped, brilliant-cut gemstone.
- 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.
- Heat Treatment The application of high heat to a gemstone in order to improve its color and clarity.
- 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.
- Iridescence Effect caused by the interference of light on thin films within the gemstone.
- 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.
- Lapidary The science and art of cutting and polishing gems to their finished state.
- 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.
- 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).
- Opaque A term used for gemstones that you cannot see any light passing through the gem. Lapis and malachite are an example of this.
- Pavilion The lower portion of a gemstone that begins just below the girdle.
- 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.
- 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.
- Refraction The bending of light as it enters a medium and slows down.
- Refractive Index A process using a refractometer to measure the speed and angle of light entering a gemstone. Very important for gem identification.
- Rough In gemology, this refers to the raw, natural state in which gems are found, before they are cut.
- Rutiles Needle-like inclusions (or foreign matter) within stones. These can produce some gem phenomena as an asterism (star) or cat's eye (chatoyancy.)
- 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.
- Single Cut Stones with seventeen facets or fewer.
- 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.
- 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.
- Step Cut A gem cut with rectangular facets along the perimeter.
- Swiss Cut A gem cut consisting of thirty-three facets.
- Table The flat top part of a gemstone. The table is the largest facet.
- 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.
- 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.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.
- Vitreous Luster The most common gem luster. This is a luster with a shiny, glass-like appearance.
- 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.