Specific gravity - 1.95 to 2.25
Hardness - 4.50 to 6.50 (this depends on minerals within the opal, and it's age)

Most opals are soft, and break easily, so care must be taken not to damage them. But boulder opals matrix of ironstone makes them very durable, once mounted.

Formed of non-crystalline silica , which over the years seeped into the strata and bound into what you will see as an Opal.

The price range of Opals are as varied as the stones themselves: $200 to $500 per carat for crystal opal.

In descending rank of value:

Black Opal
The rarest and most valuable type of Opal, it's beauty is startlingly dramatic Often found as a bar(or bars) of various colors with dramatic "flash". Black Opals are mined at only two areas on the planet - Lightning Ridge and Mintabie, Australia.

Boulder Opal
Boulder Opal is only found in ironstone which is a natural part of the stone, offsetting and enhancing it's remarkable colors. It is mined in Queensland, Australia.

White/Precious Opal
With a white or light background, the original "fire" opal from South Australia - beautiful reds, greens, and blues are embedded within. Many people don't realize that White/Precious Opals aren't the only type!

Triplet Opal
A thin slice of colorful light opal is glued to a layer of dark opal "potch" and covered with a crystal quartz capping. A doublet is similar to a triplet, minus the quartz capping.