Matrix when referred to opal can be a number of different things. The most commonly known type of opal matrix is Andamooka Matrix.
Andamooka matrix is a treated type of opal. The color will only be visible when it has been treated by boiling it in sugar and then using acid to treat it.
Even though matrix opal can look beautiful the beholder, don’t be fooled that it is worth the same as natural solid opal. I have seen many people invest in matrix opal and not know what they buying and paying too much. Andamooka will be a very 2 dimensional type of opal with no depth of play at all.
Another type of matrix is a substance that is found in some opal mining fields in Lightning Ridge. The substance is a whitish cap that nobby opal can form on it. This matrix also can but not always crack the opal. It is a very unhealthy looking rock with the silica spheres stacked very unevenly. The best thing to do for matrix on nobby opal is to cut it off as much as you can to reduce the risk of decay (just like tartar on your teeth).
Matrix opal also refers to boulder opal having many paisley and squiggly lines of opal color going through the ironstone. If you go to Yowah in Queensland’s boulder opal fields you will find this a common occurrence.
On a whole matrix opal can be very beautiful and inexpensive to the consumer and may be a great way to mass-produce jewelry.