Agate - Fine-grained with many colors of crystallized silica  and banded, striped, or mosslike coloration.

Amorphous - Having no definite form.

Black Opal - Rare form of mineral opal found only in Australia.

Cabochon - A gem or bead cut in convex form and highly polished, but not faceted. Also, a style of cutting.

Crazing - Tiny cracks on the surface.

Crystal Opal - Transparent with flashes. Highly valued due to the brilliance of its colors and the fact that many layers of color can be seen within the stone.

Crystalline - Composed of or resembling crystals.

Direction - A measure of opal value.

Dopping - Attaching a gem to a wooden extension by means of adhesive wax in order to polish or facet the stone with greater ease.

Doublet - A manufactured opal gem consisting of two layers: Opal and obsidian or ironstone.

Fire (or pinfire) - A measure of an opal's color or iridescence.

Fire Opal - A translucent or transparent mineral opal found mainly in Mexico.

Geothermal - Of or relating to the heat of the Earth's interior.

Mohs' Scale - A scale of hardness for minerals in which 1 represents the hardness of talc and 10 (sometimes 15) represents the hardness of diamond.

Organic Opal - An opal formed from the chemical petrifying of organic materials such as wood or seashells.

Potch - Crusty mineral coating on naturally occurring opals.

Quartz - A mineral SiO2, silicon dioxide, that occurs in crystals or crystalline masses.

Silica - A mineral SiO2, silicon dioxide, that occurs in crystalline or amorphous masses.

Synthetic Opal - Man-made opal.

Triplet - A manufactured opal consisting of three layers: Clear quartz, opal, and obsidian or ironstone.

White Opal - A common form of gem quality opal, usually white or milky in color with bright pinfire flashes.