Opals, the queen of gems, are found in various countries of the world, but the most famous are from well-known locations in Australia. Black Opal comes mostly from Lightning Ridge; Boulder Opal is found mainly in Queensland, and White Cliffs; Crystal Opal comes mainly from Coober Pedy. Lesser-known opal types come from their namesake areas – Mintabie, Yowha Creek, Andamooka, and elsewhere. Ethiopia is another large producer of crystal opal. Mexico produces fire opal and jelly opal. A form of boulder opal is found in Spencer Idaho, and opalized petrified wood deposits are found in Virgin Valley, Nevada.
All opal contains various amounts of water and therefore are sensitive to cracking as they dry out after being mined or cut. The opalescence responsible for the brilliant colors is caused by tiny spheres of tightly packed silica which produce shifting colors much like a glass prism produces the colors of the rainbow. The size of the silica spheres determine the various spectra of colors. The smaller packed spheres produce the reds and oranges while the larger spheres produce the greens and blues. This play of color shifts with the movement of the stone as light hits it at various angles. The color-producing zones of opals areas are usually found in the form of seams running through matrix, also called “potch”. Black Opal, for example, is made up of seams of color running through black potch. The opals are cut in such a way as to leave a dome of color sitting atop the dark potch base.
Boulder Opal, however, is actually prized for its show of seams of color surrounded by hard ironstone. Over millions of years natural fissures in ironstone are filled with silica, which opalizes. This leaves the ironstone coated on both sides of the iron matrix with the colored opal like frosting on a cake. Many boulder opals are sold in this form after the iron stone matrix is shaped into an oval or free form. Other times, they are cut much like Black Opal, with an opal “face” and the ironstone acting only as backing.
The thinner seams of opal can still be cut in layers and adhered to an underlying black stone base. The colored top is then cut flat and a crystal cap is adhered over it. This presentation of opal is called a triplet. When the crystal cap is put over solid opal without potch, it’s called a doublet. In both these presentations, the crystal cap magnifies the opal’s colors. The triplets and doublets do not command the high prices that natural single-layer opals find, although they can still be quite beautiful.
Our opals are split into designations based on their general appearance: Black Opals, Boulder Opals, White Opals, and Crystal Opals. If a particular opal comes from a famous mine, it will be listed under “Origins” rather than categorized by this information. This allows you to shop by the visuals of the stone. Here’s a guide to of the types of Opals we carry:
Black Opal – Dark grey to black body tone. The dark background adds to the vibrancy of color and thus value.
Boulder Opal – Boulder opal is split into two categories on our site, Matrixed and Unmatrixed. Unmatrixed stones appear complexly opalized from the front, with the natural ironside only visible from behind. On Matrixed Boulder Opal, the ironstone is visible from the front, sometimes with the opalized streaks running like rivers through the polished fissures of the matrix. Very exciting type of opal.
Jelly Opal – Clear to Red body tone, translucent or transparent, with or without fire. Available faceted or in cabochon. As all of our Jelly Opals originated in Mexico, we also refer to them as Mexican Opals.
Crystal Opal – Pale, translucent or transparent body tone. One way to check for the crystal nature of this opal is to hold it against writing. With a truly clear crystal opal, you’ll be able to read through the stone!
White Opal – White to light body tone or milky opal.
The fire of an opal, a colorful mass of glistening silica, comes in almost innumerable different patterns. Here at GemsbyGerald, we try to limit our categorizations of patterns to the most frequent forms, cutting down on confusion and frustration for our customers. Here are some examples:
Straw – Small and thin multiple lines of color next to each other.
Flagstone – Large, distinct blocks of color.
Ribbon – Multiple rolling flashes which line up in different sections moving next to each other and in succession.
Rolling Flash – A large section of color in which a bright flash rolls across a section of the stone as you move it.
Broad Flash – Large sections of color which flash brightly at certain angles.
Pinfire – Small dots of color sparkling like stars.
Floral – Similar to Pinfire, but with larger patches of color, resembling flower blossoms.