Citrine: A variety of crystalline quartz that is found in shades of yellow, red-brown or orange-brown. Most come from Brazil. The name itself is derived from the French word, citron, which means lemon, the color that this gem generally displays.

Cleavage: The propensity of crystalline minerals to split in one or several directions along, or parallel to, certain planes.

Cultured Pearl: A pearl produced through an artificial process that mimics the organic process by which a natural pearl is created. An irritant such as a bead, grain of sand, or piece of mantle tissue is inserted by human intervention into the body of a mollusk, and becomes the nucleus of a pearl once that mollusk secretes nacre to cover the irritation.

Crystal Structure: Natural plane surfaces in a crystal that are symmetrically arranged, and by their form express a specific internal structure that is the outcome of atomic arrangement.

Emerald: One of the most valued of all precious stones. The emerald is a type of beryl with a signature color ranging from pale green to the highly sought-after dark velvety green. Elements of chromium in the crystal produce the color, while inclusions in the stone create what is called the jardin, or garden, of the emerald. The recognized emerald cut is essentially a rectangular shape with step-cut sides and chamfered corners. Clear resins are often used to permeate the open fissures in some emeralds, while hardeners are added to solidify those resins, ensuring the permanent clarity of the stone.

Fracture: This determines the quality of a surface, obtained by breaking a crystal in a direction other than that of its cleavage.

Garnet: A dark red mineral, actually one of six main varieties of gemstones related by similar chemical composition. Used by the ancients as inlays, formally cut en cabochon in Victorian jewelry while backed by gold or silver foil, they are today usually found in step cuts.

Hardness: The ability of a stone to resist any abrasion short of cleavage.

Imitation Gem: Cheap materials, such as glass or plastic, used to mimic the quality and structure of real gems.

Moonstone: A version of transparent or translucent feldspar, customarily cut en cabochon. The stone possesses a sheen known as adularescence, which comes from the stone's alternating layers of albite that diffuse light over the dome. The finest varieties display a bluish sheen on their surface, while the commoner versions with thicker layers of albite produce a whiter sheen.

Opal: A gemstone whose chemical composition is an amorphous, gelatinous, hydrous silica containing varying degrees of water and traces of metal oxides. The two varieties are the iridescent Precious Opal, known also as the Noble Opal, and the milky white Common Opal. The opal, like the moonstone, is customarily cut en cabochon. However, the beautiful stone known as the Fire Opal is often carved, engraved or faceted.

Optical Properties: The ability of gems to handle light. These properties include refraction; transparency; dichronism, which is the ability of gems to display two separate colors when viewed in different direction; and fire, a gem's display of prismatic colors.

Peridot: A gem mined in antiquity on the Egyptian island of St. John, in the Red Sea. It is a golden-green variety of a mineral called olivine, and includes hues from leek-green to yellow-green. Because of its distinctive green color, this gem has alternately been confused with emerald and chrysoberyl. Though customarily faceted, peridot has also been polished in a process called Tumbling, or sometimes set in its natural, raw form.

Ruby: One of the great precious stones, usually more costly than a diamond of the same size. It is a red variety of a transparent corundum. Trace quantities of oxide of chromium in the stone account for the various shades of red that range from pink to deep red, or to the dark red-purple hue known as Pigeon's-Blood. Rubies contain minute, irregular inclusions, while certain stones exhibit an optical phenomenon call Asterism, in which a star-like figure is seen in some crystals by reflected light. Rubies are usually set in brilliant or mixed cuts.

Sapphire: A precious gem derived from a form of transparent corundum, with a color range from pale cornflower-blue to deep blue, or to the highly preferable Kashmir Blue. Sapphires with colors other than blue are generally referred to as fancy sapphires. Those colors can vary from white, yellow, green, pink, purple, brown and black. The gems are very hard, with a vitreous luster, while minute amounts of oxide of iron and titanium in the stone probably account for the distinctive blue of the sapphire. Their colors are often enhanced through heating.

Synthetic Gem: A term given to a manufactured gem that duplicates a natural gem chemically, physically and optically.

Tanzanite: A transparent variation of the mineral, Epidote. Though tanzanite in various colors has been found, the gem's traditional color is a rich violet-blue. Heating the gem to the desired hue is the process by which that particularly gorgeous tint is achieved. The transparent varieties of the gem are usually faceted, while the cloudier versions of tanzanite are often cut en cabochon.

Topaz: A gemstone varying in color from canary-yellow, orange-yellow, pale blue or green, pink, gold-brown or sherry-brown, and colorless. The stone is often irradiated and then heated to a rich sky blue, which is permanent and in no way harms the gem. The stone is extremely hard though extremely breakable, having a strong cleavage. It possesses a vitreous luster, as well as double refraction, low dichroism and color dispersion. Often this gemstone, in its colorless variety, is mounted as a mixed or a brilliant cut.

Tourmaline: A gem with an intricate chemical composition that produces a variety of colors, including blue, red, pink, green, brown, yellow and black. There is also a highly prized colorless variety. Heat is used either to lighten or enhance the colors of certain tourmalines. In many cases the tourmaline is known for its ability to reveal different hues from different angles, a characteristic known as dichroism. In addition, fibrous inclusions in certain stones create an optical formation commonly called Cat's-Eye. The tourmaline's colors are often heightened by the specific method of gem cutting used on them, which more often than not with this stone is the mixed cut.