Unique Birthstone Jewelry
Opal is the modern October birthstone and the accepted gem
for the 13th wedding anniversary. See other October
birth stones: Traditional, Contemporary,
Mystical, Ancient, Zodiac and Star signs.
Most opal is 50-65 million years old, dating back to the
Cretaceous period when dinosaurs roamed the earth. Opal formed
as silica from decomposing rocks mixed with ground water which
formed a silica gel that collected and hardened in underground
cavities and fissures. Opal's chemical formula is SiO2 .nH2O.
See more opal
There are two distinct types of opal, common and precious.
The way the silica particles form determines which type. In
precious opal, silica particles are packed in regular rows
and layers. Moving the stone causes light to diffract, or
split, as it grazes the opal surface. This light diffusion
shows iridescent flashes of green, blue, aqua and sometimes
yellowish or red colors which are referred to as "fire".
Color and Varieties:
Black Opal comes from the Lightning Ridge region of Northern
New South Wales. It is the most rare and valuable opal. Fine
quality black opals often cost more per carat than diamonds.
The term black opal is misleading because the opal is not
actually black, but instead has a very dark base. This dark
base enhances the brilliant colors known as "fire".
Boulder opal occurs in the boulder opal belt, an area between
the New South Wales border and northwest Queensland. Boulder
opal forms on a dark ironstone base (the host rock) and occurs
as a thin uneven layer adhering to the ironstone. Because
of the uneven layers, sometimes part of the ironstone is visible
on the surface of the stone. Queensland Boulder Opal accounts
for a small percent of the world market, and because of its
brilliance and rarity can bring up to $1000 a carat.
Boulder opals are graded into specific groups (listed below)
which have been defined by the Australian Gemological Association:
Boulder black opal
Boulder crystal opal
Boulder light opal
Boulder matrix opal
Seam and vein opal
opal is the most common type of opal found and refers to both
the crystal type opal (translucent) and the milky opal (opaque).
It is the most common variety of opal and is generally less
expensive than gem quality light opal. Brilliantly colored
light opals may be quite expensive with a value exceeding
some Black and Boulder Opals.
Brilliance or luster is one of the primary factors that determine
an opal's value. An opal with strong intensity and color play
adds more value.
Rumor and Legend Surrounding Australian
A long standing rumor is that boulder opals are porous and
will absorb moisture, such as perspiration but an Australian
opal is not porous and will not absorb moisture. To change
the water content locked in an opal's tiny voids it must be
heated extreme temperatures. Opal's hardness on the Mohs
scale is 6.5.