During the Cretaceous period (65–140 million years ago), the deserts were an inland sea teeming with life. The sea gradually receded and laid the foundations of the silica that would form today's opal. During the mid-Tertiary period, changes in Earth's climate caused quantities of soluble silica to be released from the sediment, which found its way down through cracks and faults and eventually hardened over time. This hardening continued to form the common and expensive opal.
The vibrant array of colors in opal comes from the uniform arrangement of silica particles. The size of the particles determines the color as light refracts through them.
Unlike many other fire opal rings, opal does not occur in long veins or concentrations. Instead, small clusters can be spread over a large area and split into gems or gems and common. Opal comes in many varieties, but expensive opal represents only a small percentage of the total mined.
Black Opal:
Black opal is the rarest and most valuable of all fire opal necklace and generally occurs as a rod of various colors in a dark body.Black opals from Lightning Ridge, Mintabie and Andamooka in Australia make up 99.9% of all opals in the world.
Boulder Opal:
Boulder opal, found in the fields of Queensland, is classified as a solid opal and occurs as thin veins of precious opal in the cracks and cavities of ironstone. During processing, the stone is cut, leaving the natural host rock as the base. Black opal engagement ring soccurs either as a solid piece of color on top of the ironstone, or it appears as flashing flecks of color throughout the stone.
Light opal (white or milky):
A full range of colors can be found, with a background color of either white or light blue.
Crystal Opal:
Colors come to life when looking at a dark surface.
Picture of Opal:
Image opals are so called because their pattern forms an image of an object or person. A good imagination is sometimes required to visualize an object/person.
Fire Opal:
The term "fire opal" is commonly used to describe the clear orange crystal opal that originates in Mexico, some of which have play of color, although many have only an orange or reddish base with no play of color.
Opalized Fossils:
In the layers where opal mining takes place, the remains of the world more than a hundred million years old are buried, brought to the surface by miners looking for gems. Some of these fossils were opalized, a process in which silica-rich waters gradually replaced the organic material.
Common Opal:
Common opal is classified as a non-precious opal, mostly opaque and showing no play of color.
Matrix Opal:
Matrix opal is when the opal is infused into the rock in which it was formed. Veins of precious opal penetrate the sandstone or ironstone filling the holes and fissures of the host rock.
Synthetic Opal:
Synthetic opal, as the name suggests, is made in a laboratory and has a similar structure to expensive opal.
The following can be made to differentiate between natural opal:
A. Synthetic stones show brighter colors and larger color spots;
b. In synthetic opal, the color grain boundaries are highly irregular;
C. Synthetic opal has a distinctive snakeskin pattern;
d. The material exhibits a more ordered range of colors because the complex patterns of natural opal cannot be duplicated.
Raw Opal Packages:
Raw opal packs are sorted into three grades: Top, Middle and Low.
Each top deck has a king stone (the best stone in the deck). Color is the primary criterion for classification.
You can buy raw opal in several different conditions:
A. Mine Run - Straight from the miner himself.
b. Off Cuts - The miner removed all the tradable opal and you sold what was left. With off cuts, you can usually tell what you will be able to cut.
C. Rubbing - After removing most of the waste, the opal stones were cut and ground into basic shapes.
Andomooke Matrix:
A more porous opal found in mining fields. It is quite pale from the ground, but when treated with carbon dye it looks like a true black opal.
Yowah Nut:
This rare and unusual opal, up to 5 cm in diameter, looks like a round or oval nut and is an iron stone.
Opal forms in the nut either as a solid core or, more commonly, in concentric layers infused with the host rock. The nut must be split in half before the contents are revealed and can be split to create mirror images.
Opal Doublets:
Opal doublets, as the name suggests, is a thin layer of precious opal glued to a backing, usually a piece of black common opal or ironstone. Made because opal is too thin to make a solid stone, doublets are a cheap and inexpensive way to buy a colored piece of expensive opal without the cost of a solid.
