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.

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

see density

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.

[Source: www.gemselect.com]