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Glossary of Rock and Mineral Terms

What do the words used to describe rocks and mineral mean?

Everybody loves the beauty of natural rocks and minerals. However, not everybody knows the meaning of all the words or terms used in the rock and mineral world. I’ve prepared this information guide to help those people understand more about the rock or mineral specimen they are thinking of buying.

Here are just a few of the common words used to describe rocks and minerals and their definitions:

First off the most important: Mineral is a naturally occurring solid substance formed by natural processes, with a specific chemical composition. Rock is a naturally occurring solid substance of one (but usually at least two) or more minerals. Gem or gemstone are terms used loosely to describe the best minerals and rocks. And Stone is really not a scientific term but is used often in the commercial trade.

The fields of study and the people who study them include: Gemology/Gemologist the study of gemstones, Geology/Geologist the study of substance that makes up earth, Mineralogy/Mineralogist the study of minerals, Paleontology/Paleontologist the study of prehistoric life and fossils. Lapidary means “concerned with stone” and is used to encompass the entire field as well as the art of cutting stones and the person who does the cutting.

Other mostly descriptive words or terms used to describe rocks and minerals are:

Botryoidal  This literally means “cluster of grapes” and is used to describe a rock that has a bumpy or grape like surface.

Cabochon (also known as Cab for short)  A stone cut for jewelry. It is usually rounded (or domed) and polished on top, and either flat or slightly rounded on the bottom. Cabochon stones can be calibrated (cut to a certain size/shape) or freeform (an abstract shape). This form of cutting is usually used for opaque or translucent stones, but is sometimes used for transparent stones with too many inclusions to make a good faceted stone (faceted is the way diamonds are cut with tables and angles). Cabbing or Cabber are often used to describe the action of cutting a cabochon and the cutter. 

Chatoyant/Chatoyancy  Chatoyancy is a lustrous, cat's eye effect seen in some stones and minerals. In chatoyant material, light is reflected in thin bands within the mineral or stone. Chatoyancy arises either from the fibrous structure of a material, or from fibrous inclusions or cavities within the stone. The name comes from the French word for "cat's eye," because it resembles the slit eye of a cat.

Druse, Druzy or Drusy  Druse is the correct word and it is a layer of crystals that formed within a cavity of rock. But people often use Drusy or Druzy.  The inner cavity of agate geodes is often lined with druse of tiny sparkling quartz crystals. These crystals reflect (or appear to be) the color of the rock underneath. Sometimes in the rock trade, minerals that have a fine layer of tiny sparkling crystals can be referred to as druses, such as Cobalto Calcite being called Pink Drusy, or Uvarovite Garnet being called Green Drusy. These minerals can sometimes be cut into designer cabochons for jewelry.

Flaw  A flaw is an imperfection in a mineral or rock. Flaws include: cracks, chips or “dings”, and some natural inclusions or fractures. A flawless stone is called "clean”. Flaws can sometimes greatly reduce the value of a stone.

Fluorescence  Fluorescence is when visible light is emitted from an object during exposure to invisible radiation. Ultraviolet light can produce vibrant red, green, blue, yellow, and other colors in a variety of minerals. Some 500 minerals are fluorescent.

Fracture  A crack in a mineral or rock that can be natural or man made. It can be  naturally healed, meaning something happened, perhaps the ground shifted and cracked, and then the crack was filled with natural material.

Hardness  The Mohs scale of mineral hardness rates the scratch resistance of various minerals on a scale of 1 to 10 It rates the ability of a harder material to scratch a softer material. The scale runs from 10-Diamond, the hardest substance known to man, to 1-Talc, the softess. Though diamond is the hardest, jade is actually the toughest, or least likely to break.

High Grade/Mine Run  High Grade material is exceptionally good. Also to “high grade” a mineral collection is to sort out the best specimens. Mine Run is usually used to state that the material is sold as it comes out of the mine. The good and bad mixed, it has not been sorted or high graded.

Iridescent/Labradorescence  An iridescent or labradorescent object displays many lustrous, changing colors. Iridescence and labradorescence are caused by dispersion of light in cracks and flaws resulting in a rainbow-like play of color (as often seen in an oil slick or a soap bubble). The colors tend to change as the angle of view changes. The word Iridescent comes from the Greek word “iris”, which means rainbow.

Inclusion  An inclusion is a particle of foreign matter contained within a mineral or gemstone. Inclusions can be solid, liquid, or gaseous. A water filled pocket is called an enhydro. Organic inclusions are only found in Amber. Inclusions are natural and not always considered flaws.

Lapidary Lapidary is the world of stones and gems and the work involved in engraving, cutting, or polishing them.  The person who cuts and polishes stones is also called a Lapidary.

Opaque  Opaque means no light passes through the material.

Psuedomorph  When one mineral completely replaces another, but retains the same outer shape of the replaced mineral.

Rough  Is rock as it comes out of the ground and has not been worked yet.

Synthetic  Is the term given to stones that are man-made.

Stabilized/Treated  An impregnation of a resin to make the rock harder or more durable, so it can be cut easier. This does not effect the attributes of the stone-such as its color. Treated can also refer to process done to change the stones attributes, such as heat-treating, irradiation, dyeing, etc.

Translucent/Transparent  Translucent materials allow some light to pass through them, but the light is diffused. The material appears to glow. Transparent means clear or light shines freely through the material.

Vug  This is a natural hole or hollow area in rock. Also called a pocket or cavity.

 

Learn more about The Origin of Rock Names here.

And check out all the Rock and Mineral Information available here.

 

Finally take a list at our Rocks and Minerals list for more specific information about a specific rock.