Gem of the Month

Each month this section will feature either a topic of interest to gem lovers or one special gemstone with background on the material and its value.

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July, 2002

Boulder, Yowah Nut and Matrix Opals

Precious opal which is found as seams or filling cavities within a matrix rock is referred to by one of the above terms. These types differ primarily in the location of the opal deposit and the nature of the matrix, but all of them are fashioned into finished gems with the matrix as an integral part of the piece.

It was not until the 1990's that these types of opal became popular. Some analysts regard the growth in their acceptance as an offshoot of the tremendous increase in prices of white and particularly black, opal in the 1980's. Utilizing boulder opals made it possible to have the dramatic colors and contrasts of black opal at much more affordable prices. Today, rather than being viewed as a cheaper stand-in for the "real thing", these opals are appreciated for their own special characteristics.

All forms of precious opal, including the boulder types, get their color play from their submicroscopic internal structure of tiny silica spheres, stacked on top of each other like oranges in a crate. This creates a sort of diffraction grating or prism which causes white light to separate into colors. The size and regularity of the spheres determines the hue, intensity and pattern of the colors we see.

With the dark ironstone matrix intensifying and increasing contrast in the opal layer, boulder opals are particularly colorful -- this matrix creating the same effect as the black backing on an opal doublet. Boulder opals have thus been described as "Nature's doublets" and are generally cut with a solid or near solid layer of opal over the matrix. Not only does the ironstone matrix improve the color of the opal layer, it also increases its stability. Boulder opals are less likely to crack or craze than most other types of opal. Since each piece is so unique in size, shape, and pattern, they lend themselves much more to custom designs than to calibration and use in mass production. They are produced exclusively from Queensland, Australia.

Also produced in Queensland is "Yowah nut" opal, named for the place they are found and the nut-like shape of the nodules. The ironstone forms the "husk" and the opal is found as the "meat" of the nut.

Matrix opals come from a variety of places such as Andamooka and Queensland as well as areas in Mexico. In these stones, the opal and matrix are intertwined and the matrix be it light or dark is highly visible in the finished gem.

Opals must be worn with care and are not recommended for daily use rings or other jewelry that will receive hard knocks. They should be protected from harsh chemicals and high temperatures and ultra-low humidity (as found in safety deposit boxes). Storing opals in water is not necessary, or even advisable, and they should never be stored in oil. Wearing them is the best preserver of opals as the skin provides a constant, moderate temperature and moisture level. When cleaning is necessary, mild detergent and a soft brush are best.

Although white and black opals have been both simulated (Slocum Stone) and synthesized artificially (Gilson and others), boulder, Yowah nut and matrix opals have not.

These beautiful and highly individualistic gemstones deserve a place in every gem collection and jewelry box.


Value Factors

As with any opal the intensity, pattern and coverage of the color is all important in determining value. Black opal colors increase value as do strong broad flash patterns. Although most matrix opals are available at modest prices, with Yowah nut opal usually slightly higher, boulder opals can be quite expensive. Top pieces have sold for $50,000. You don't need to win the lotto to enjoy a beautiful boulder-type opal, though, as they are available in all price ranges.


Gemological Properties

Makeup: Hydrated Silica Gel

Hardness: 5.5 - 6.5

Refractive Index: 1.44 - 1.46

Density: 1.95 - 2.20

Crystal System: amorphous

Luster: vitreous


Stones Currently Available:

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Go to: Homepage -- what's new in faceted gems -- what's new in designer cabochons and gem carvings -- gem of the month -- gem of the month archive -- birthstone of the month -- key to all the codes used on the ACS site -- definitions of terms used on the ACS site -- how to order -- about ACS -- about the ACS cutters -- settings for these gems --faceting information -- purchase UltraTec equipment