Matrix opal is a treated opal that comes from in Australia. It is defined as opal in host rock. The main field that produces this matrix opal is Andamooka in South Australia. The field was discovered in the 1940’s.
The opal usually has little or no color unless it is treated. There are a few different processes. One process is the opal is boiled in sugar to bring out the color in the stone. Matrix opal usually has no depth and you cannot see through it.
Don’t mistake matrix opal with black opal because the price difference is massive. Matrix opal will not be worth anywhere near the price of black opal due to the fact it has been treated. Black opal on the other hand is not treated. Matrix opal can be very pretty and is still considered a gemstone.
Boulder Matrix most often found in Yowah, Queensland, is another type of opal that is not treated and has small deposits of opal color in the ironstone giving the opal a look similar to Andamooka Matrix. It is also defined as opal in host rock.
I have seen people who wanted to invest in black opal, buying matrix opal thinking it was black opal and spending a lot of money on something that is not worth what they paid. Andamooka matrix which has been treated should always be disclosed as a treated opal.