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An intimate mixture of precious opal and host rock.
Author: Hobart M. King, Ph.D., GIA Graduate Gemologist
Andamooka Treated Matrix Opal: The cabochon above is an example of a limestone matrix opal from the Andamooka area of Australia. Tiny occurrences of opal are distributed through a light-colored limestone host rock. The host rock is darkened in color by smoke or dye to provide a greater contrast with the opal.
What Is Matrix Opal?
Matrix opal is a variety of precious opal (or play-of-color opal) in which the opal is intimately mixed with its surrounding host rock. The precious opal occurs in the host rock in vesicles, as tiny pore filings, or as small areas of host rock replacement. Many people call this material "Type 3 Opal".
This intimate mixture makes a gem material that looks like the host rock, but with patches of precious opal flashing within. The cutter must study the rough and determine a cutting orientation that will produce a cabochon exhibiting a nice presentation of opal and host rock in the face-up view.
Matrix opal is produced at several localities around the world. Some of the best-known are in Australia, Honduras, and Mexico. A few examples of matrix opal in various types of host rock are shown on this page.
Andamooka Magnification: The photograph above is of a small area of the Andamooka opal shown at the top of the page, viewed under a microscope. It shows in better detail how patches of precious opal are in intimate association with the host rock. The width of the field of view is about three millimeters.
Spectacular Color in Matrix Opal: The cabochon above is another example of treated Andamooka matrix opal. It shows spectacular patches of vivid blue, yellow, red, orange and green color. The photo is a Creative Commons image by Dpulitzer.
Matrix Opal in Limestone
Limestone is a soluble rock that can be replaced or silicified by the actions of underground waters. Precious opal can form in tiny voids within the limestone. It can also form as groundwater dissolves some of the limestone, and immediately precious opal precipitates in its place. This chemical process is known as "replacement" or "silicification".
One of the best-known examples of matrix opal in limestone is the Andamooka Opal of Australia. The opal occurs in a light-colored limestone, which opal cutters often darkened by treatment with smoke, sugar solutions, or dye. The result is a dark matrix of black, dark gray, or brown which provides a contrasting background to the flashes of precious opal that are distributed through the rock. The beauty of a nice piece of Andamooka surprises many people when they see it for the first time.
When Andamooka or any other matrix opal has been darkened or colored by treatment, it should be labeled as "treated matrix" opal when offered for sale. Many people believe that it should not be called "Type 3 Opal" because of this treatment.
Louisiana Matrix Opal in Sandstone: If you look closely at this cabochon, you will see rounded sand grains with some of their interstitial spaces filled with precious opal. The cabochon was cut from a small slice of the Catahoula Formation found near Leesville, Vernon Parish, Louisiana.
Matrix Opal in Sandstone
Occasionally matrix opal will form in a porous sandstone. The opal precipitates in the interstitial pore spaces between the sand grains, forming a "cement" that binds the rock together. An example of matrix opal in sandstone is "Louisiana Opal", which is found in the Catahoula Formation near Leesville, Vernon Parish, Louisiana.
This material started out millions of years ago as loose sand grains deposited as a sediment. Then, groundwater moving through the sediment delivered small amounts of dissolved silica that, over time, precipitated as precious opal between the sand grains. People found this sandstone, were surprised by the play-of-color in the rock matrix, and cut it into cabochons for use as a gem.
Honduras Black Opal: The cabochon shown above is an example of matrix opal in basalt from Honduras. The basalt contains tiny vesicles and cavities that are filled with precious opal. The black color of the basalt makes the play-of-color more obvious.
Honduras Black Opal
An example of matrix opal in basalt host rock is found in Honduras. There, a sequence of stacked basalt flows have extremely small vesicles and other pore spaces that are often filled with precious opal. The precious opal contrasts beautifully with the black basalt matrix.
These deposits, near the community of Erandique, were mined at dozens of locations long before commercial mining of opal occurred in Australia. There is a lot of opal in this geographic area, with nearby locations having boulder opal in rhyolite and andesite host rock.
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