Matrix opal is generally referred to as boulder matrix which is opal that has formed in ironstone. But also can be matrix opal formed in sandstone at Andamooka
Most is matrix is from boulder opal fields in Queensland and this can cover thousands miles so mineral composition can vary from each area
Generally the matrix in Queensland is opaline silica as an infilling of pore spaces in silty claystone or ironstone
Bright opal colours occurs as a network of veins or infilling of voids or between grains of the host rock ironstone
Matrix formed in andomooka is in layers of limestone so not as solid or a s stable generally as Queensland boulder matrix. But ti also forms as Rainbow Opal Matrix.It used to be called concrete opal that when cooked displays bright colours
Some cutters block these matrix in square shape and make round beads out of them a s the colour is mostly impregnated in most of the opal and not on the surface a s normal boulder forms.
Some deposits are strong blues to green colours and other deposits are sunset orange to red fire colours and it is not till the rock is opened that you get to see the colours
Matrix opal is also known as Type 3 Opal and is also found in Honduras and called Honduras black opal. Australia matrix opal for opal fields Queensland form Yowah ,Koroit to Winton and Quilpie produce some spectacular pattern opal.