There are four types of naturally occurring opal; black, boulder, crystal and white.
Black opal is principally found at Lightning Ridge in New South Wales, Australia. This magnificent gemstone is the most valuable form of opal. It's dark background color, usually black, blue, brown or gray, sets the spectral colors ablaze much like a storm cloud behind a rainbow. Black Opal is a gemstone that has had an important effect overseas, as a product of Australia. It requires a precise meaning so that the quality of this gem can be meaningfully established. Sometimes off-colored white opal has been passed off to a visitor as being a black opal! The following points can be considered in the problem of recognising a genuine black opal:

1. Black refers to the body color of natural, solid, precious opal. A clear transparent layer of precious crystal opal naturally formed on black potch opal may transmit that base's darkness through its own substance and so assume the quality of being black. This is black opal too.
2. Black Opal is not a term applicable to matrix opal, whether naturally black or artificially stained, nor to Queensland boulder opal.
3. No opal doublet should be described as black opal, even though the veneer of noble opal may have come from a black opal.
4. The categories of black, semi-black, and light-to-grey opal cannot be inflexibly defined. When does a stone grade from black to semi-black? Your commonsense can dictate this and, if in doubt, put it in the lighter category.

Why, we might ask, is black opal black? The reason for blackness in an opal is the presence of impurities of iron oxides, scattered like fine dust through the substance, in sufficient quantity to impart a jettiness of color. Black opal from Lightning Ridge has carbon along the pseudo-crystalline boundaries. The base color of white opal is a property of the structural imperfections in the stacking arrangements of the basic silica micro-spheres that compose opal; these imperfections scatter and diffract white light. Black opal absorbs most of the white light that impinges upon it, save for that fraction which is diffracted as glorious colors.

Boulder opal is found sparsely distributed over a wide area of Australian ironstone or boulder country where the opal in fills cracks and crevices in the ironstone boulders. Opal bearing boulder is always cut including the host brown ironstone. Boulder opal is in very high demand and extremely precious. Boulder opal is usually cut to the contours of the opal vein creating a baroque wavy surface and is often freeform and irregular in shape, making boulder opal unique and exclusive among it's peers.

Crystal opal is transparent and is pure opal (hydrated silica.) It typically has sharp clarity of diffracted color visible from within and on the surfaces of the opal. When held out of the direct light, crystal opal displays some of the most intense opal color. This is the type of opal used in opal inlay jewelry, which has the base of the setting blackened before the precision cut crystal opal is set into it.

White opal is the most common type of precious opal and is translucent with a creamy appearance which dominates the diffracted colors. All of the opal fields produce white opal with most of it being mined in Coober Pedy.