Semi-Black Opals: Opals with a mid-gray to
dark background are classified as Semi-Black Opals.
Crystal Opal: A dark, translucent or semi-translucent gemstone with a
The classifications between
opals vary between dealers. Comparison stones are needed to understand
the various depths of body colors and even so it can be difficult to
distinguish between some black and semi-black opals.
black opals were first discovered in Australia in 1903, they were not readily
accepted, many people assuming they were treated opal or white opal glued to
black onyx. Some jewellers considered them to be valueless.
grade black opal can sell for USD$15,000 per carat. Some exceptional
stones have been sold for over USD$20,000 per carat.
Opal: Opal that is still attached to rock, usually ironstone, in which it
was found. This opal is found in boulders whose cracks and spaces have
been infiltrated with silica. Boulder opal often sells for several
thousand dollars per piece, but attractive gemstones can be purchased for a few
hundred dollars. For people who cannot afford a colorful black opal,
boulder opal can be a good alternative. It costs less because the stone
is not solid opal - it is a layer or opal naturally adhering to a rock base.
Opal or Opal-In-Matrix: Stones with lines or spots of opal randomly
scattered (mixed) throughout the matrix (host rock in which a mineral, fossil
or pebble is found). The most common type of matrix opal found on the
market is a porous opal from Andamooka, Australia which is often dyed black to
simulate black opal. Yowah Opal, another type of matrix opal is
completely natural and is mined in Yowah, South NSW. The patches
and lines in its ironstone mtarix often forms distinctive patterns.
Matrix opals sell for much less than boulder opals.
Opals: A translucent to semi-translucent opal with red, orange or brownish
body colour with or without a play-of-colour. Mexico is the principle
source of fire opal. Even if it has no play-of-colour, the red and orange opal
from Mexico is named fire opal.
The most valuable
fire opal is reddish, transparent and has play-of-colour. This quality
can retail for as much as USD$300 per carat. Translucent gemstones that
are yellowish or brownish can sell for around USD$5 per carat.
Opal is sometimes
impregnated with oil, plastic or wax to improve the play-of-colour and to
prevent or disguise cracking. The plastic method is stable, but oil and
wax are not stable. There are various techniques for creating the
appearance of black opal, including smoke impregnation, backing with black or
coloured paint and treatment with dye, silver nitrate or sugar carbonized by
acid. Avoid repolishing or applying solvents to treated opals.
Composite Natural Opal
buyers seeking stones with the appearance of black opal but without the
expensive price tag might consider an opal doublet or triplet.