opal terminology

By far, most of the precious opal found in Virgin Valley occurs in the form of limb casts or in wood/opal combinations. Imagine, if you will, that a volcanic explosion knocked over the vegetation in the area, some falling into an ancient lake where it eventually water logged and sank into the ooze at the bottom. Over geologic time the lake dried and the ooze solidified around the wood. At some point layers of volcanic ash, sometimes hundreds of feet thick were laid in over the lake bottom. Water percolating through the ash became saturated with silica particles which eventually made their way into the the wood and petrified it. Voids in the wood might fill with the silica gel and which eventually sets up to become opal. Combinations of petrified wood and opal are called wood/opal. If the wood managed to rot away before the opal leached in it will leave a void in the hardened ooze (clay) in the exact shape of the original wood; in other words a casting hole. If silica gel then fills the void the result is called a limb cast.

There are hundreds of names for Opal varieties. Some names, such as White Opal and Hyalite, are universally accepted. Other names are sometimes used and some are hardly ever used. This list includes all encountered names including geographical varieties and varieties describing a particular play of color. Some Opals fit the descriptions of more than one variety.
Obsolete or rarely used names are not listed.

Variety	Description
Agate Opal	Banded variety of Opal. May also refer to Agate with alternating bands of Opal.
Amatite	Opal in the form of thick mounds, formed from hot silica-rich springs. See also Geyserite.
Amber Opal	Opal with a brownish to yellowish background color, resembling Amber.
Andamooka Opal	Opal from Andamooka, South Australia.
Banded Opal	Form of Common Opal with color bands. Synonym of Agate Opal.
Bandfire Opal	Precious Opal with play of colors in wavy bands.
Black Opal	Precious Opal with a black, dark blue, dark green, dark gray or similar darkly colored background or base color. Black Opal is the most valuable form of Opal.
Bone Opal	Opal pseudomorph after a bone.
Boulder Opal	Precious Opal from Queensland, Australia, found in the cracks of, or as coatings on, ironstone or sandstone boulders.
Cachalong Opal	Opaque, highly porous type of Common Opal.
Cherry Opal	Orange-red to bright red variety of Mexican Fire Opal.
Chloropal	Common Opal similar to Prase Opal, but with a lighter green hue.
Chrysopal	Common Opal similar to Prase Opal, but with a golden-green color.
Claro Opal	Transparent Precious Opal from Mexico with an intense red, green, blue, and yellow play of color.
Common Opal	Any Opal lacking play of color.
Contra Luz Opal	Precious Opal where the play of color is visible only when a light source is behind the stone.
Coober Pedy Opal	High quality Precious Opal from Coober Pedy, South Australia.
Crystal Opal	Transparent to translucent Precious Opal where play of color is visible on the surface and in the interior of the stone.
Dark Opal	Synonym of Black Opal.
Diatomite	Opal replacement of microscopic shells of diatoms (type of microscopic organism) clustered together. It is white, opaque, and chalky in texture. Synonym of Tripolite, Fuller's Earth, and Diatomaceous Opal.
Fire Opal	Fire Opal is incorrectly used to describe Precious Opal, or Opal with play of color. The true definition of Fire Opal is Opal with an orange to red color. If the Fire Opal displays play of color, it is more correctly known as Precious Fire Opal.
Flame Opal	Precious Opal where the play of color consists of red streaks or bands that flicker like a flame when the stone is rotated.
Flash Opal	Precious Opal with large schillers that abruptly appear and disappear as the stone is rotated.
Flashfire Opal	Synonym of Flash Opal (above)
Fossil Opal	Opal pseudomorph of organic matter such as shell, bone, and trees.
Gelite	Opal (or Chalcedony) as an accessory mineral that acts as the bonding agent of Sandstone or other cemented rock fragments.
Geyserite	Opal formed from deposition of hot water springs. Also called Perlite, Fiorite, or Geyser Opal. See also Amatite.
Gilson Opal	Synthetically produced Opal.
Girasol	Yellow or orange variety of Precious Opal in which the play of color seems to follow the sun as the stone is rotated.
Glass Opal	Synonym of Hyalite
Gold Opal	Common Opal with a golden hue.
Harlequin Opal	Precious Opal in which the play of color is arranged in a consistent harlequin, diamond-shaped, or rectangular-shaped pattern that is very vivid. Harlequin Opal is one of the rarest and most prized forms of Opal.
Honey Opal	Transparent to translucent Opal with an orange to orange-brown, honey-colored background. It may or may not display play of color.
Hungarian Opal	Any Precious Opal from Europe. However, nowadays this term often refers to any White Opal, regardless of where it was found.
Hyacinth Opal	Synonym of Girasol
Hyalite	Colorless, misty-blue, or sky-blue transparent variety of Common Opal. Usually forms botryoidal masses as well as strange and unusual forms. All Hyalite fluoresces green.
Hydrophane	White, opaque, highly porous Opal, that, when placed in water, allows the water to seep into it. This causes the stone to become transparent and almost invisible while in the water.
Iridot	Old name given to Opal for a short period of time when Opal had a reputation of causing bad luck.
Isopyre	Impure, dark red form of Opal. Isopyre was once thought to be a separate mineral.
Jasper Opal	Brecciated Jasper in which the cementing material is Opal.
Jelly Opal	A transparent Precious Opal with a gelatinous appearance and a bluish sheen. Jelly Opal may also refer to a colorless, transparent Common Opal.
Lechosos Opal	Precious Opal with a milky-white background color displaying a strong play of color. May also refer to Opal with a strong green schiller.
Lemon Opal	Common Opal with a lemon-yellow color.
Levin Opal	Precious Opal with long and thin, lightning-like flashes.
Light Opal	Synonym of White Opal.
Lightning Ridge Opal	Opal from Lightning Ridge, New South Wales Australia. Although different forms of Opal are found there, this term usually represents the high quality Black Opal found there.
Lithoxyl Opal	Wood Opal where the original structure of the tree is very apparent.
Liver Opal	Synonym of Menilite (below)
Menilite	Opaque, grayish-brown form of Common Opal. Also known as Liver Opal.
Mexican Fire Opal	Form of transparent Opal from Mexico, usually with an orange or red colors, highly desired as a gem. Although scientifically considered a Common Opal, it is rather rare and much sought after. If it exhibits a play of color, it is known as Precious Fire Opal.
Milk Opal	Opal with a milky-white color. Controversy exists whether the name Milky Opal is coined for a milky white Common Opal or a milky white Precious Opal.
Moss Opal	Common Opal containing inclusions resembling moss.
Mother of Opal	Precious Opal with bright color specks filling the pores of sandstone or ironstone.
Mother of Pearl Opal	Banded Opal used as cameos.
Mountain Opal	Opal from igneous environments. Also called Volcanic Opal.
Neslite	Common Opal similar to Menilite, but darker gray in color. It was once a popular material for sword handles.
Nevada Opal	Opal from the Virgin Valley (Humboldt Co.), Nevada.
Onyx Opal	Common Opal resembling banded Onyx.
Opal Matrix	Thin layer of Precious Opal on host rock. Small rock fragments are used in jewelry.
Opaline	Opaline is synonymous with Opal Matrix (above), but was also an old term used to describe Opal from Australia.
Opalite	Opalite has many connotations. It may refer to: an impure form of Opal Myrickite black glass a yellow-green Common Opal with black inclusions an imitation Opal produced from resin
Opalized Bone	Synonym of Bone Opal
Opalized Fossil	Synonym of Fossil Opal
Opalized Shell	Synonym of Shell Opal
Opalized Wood	Synonym of Wood Opal
Painted Boulder	Sandstone boulders with a coating of Precious Opal. When used in jewelry, this term is synonymous with Opal Matrix.
Pearl Opal	Synonym of Tabasheer
Pineapple Opal	Opal pseudomorph after Ikaite that resembles a pineapple. It is found only in White Cliffs (New South Wales), Australia. The pseudomorphed mineral was originally thought to be Glauberite, but studies now prove it to be Ikaite.
Pinfire Opal	Precious Opal with very small, pinhead-size color flashes.
Pinpoint Opal	Australian synonym of Pinfire Opal
Pipe Opal	Opal formed as a filling of long, cylindrical cavities in rock. Pipe Opals range in size from several inches to many feet.
Pitch Opal	Yellow to brown Common Opal with a pitchy luster.
Potch	Australian term for Common Opal.
Prase Opal	Green to dark green form of Common Opal.
Precious Fire Opal	Fire Opal displaying play of color.
Precious Opal	Any Opal exhibiting a play of color.
Prime d'Opal	Synonym of Mother of Opal
Pyrophane	Precious Opal in which the play of color wanders about and reappears at random. This term is sometimes incorrectly used to describe Girasol.
Queensland Opal	Synonym of Boulder Opal
Quinzite Opal	Rose to pink colored Opal. It is usually without play of color, but a few examples displaying play of color are known. Quinzite Opal is synonymous with Quinzite, Quincite, Quincite Opal, and Rose Opal.
Radiolite Opal	Common Opal of a smoky-brown color caused by inclusions of the exoskeletons of a unicellular marine organism known as radiolaria. May also be called Radio Opal.
Rainbow Opal	Precious Opal where the play of color is seen in curved bands, somewhat resembling a rainbow.
Red Flash Opal	Precious Opal with red color flashes that swiftly appear and disappear as the stone is rotated.
Resin Opal	Common Opal with a yellow-brown color and resinous luster.
Rumanite	Opal from Romania.
Seam Opal	Opal found in the seams or large cracks of rock. May also specifically refer to masses of white Common Opal containing bands of precious White Opal.
Semiopal	Term sometimes used to describe any type of Common Opal, but many times alludes to particular forms of Common Opal, such as Wax Opal, Prase Opal, etc. Semiopal is also written as Semi-opal, and is synonymous with Half-opal.
Shell Opal	Opal pseudomorph after a shell.
Slocum Stone	A synthetically grown Opal. Also called Slocum Opal.
Sun Opal	Name that describes several types of Opal. May refer to Fire Opal, Mexican Fire Opal, Honey Opal, or Amber Opal.
Tabasheer	Opal occurring as an organic byproduct. It forms by the hardening of a secretion issued from certain bamboo, forming a porous, rounded mass of Opal.
Virgin Valley Opal	Opal from the Virgin Valley (Humboldt Co.), Nevada.
Wash Opal	Waterworn Opal pebbles from alluvial deposits.
Water Opal	Synonym of Jelly Opal
Wax Opal	Yellow to brown Common Opal with a waxy luster.
White Cliffs Opal	Opal from the White Cliffs, New South Wales, Australia
White Opal	Precious Opal with a light body color (white, yellow, cream, etc.). Differentiated from Black Opal which has a dark background color.
Wood Opal	Any Opal that formed a pseudomorph after wood from a tree, and retains the original shape and appearance of the wood. Wood Opal may refer to both Common Opal and Precious Opal, but the term usually refers to large pieces of Common Opal.
Yowah Nut	Small, rounded form of Boulder Opal from Yowah (Queensland), Australia in a nodules embedded in ironstone. Closely related to Boulder Opal, it occurs most often as walnut-sized ironstone nodules containing pockets, veinings, or sprinklings of vivid Precious Opal.

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