Tutorial is ready (BLENDER 2.8, CYCLES). I’ve created a lovely way to make opals, hope you like it.
Nodes on the following screens. The material uses underlying mesh geometry,
created from 2 different particules on an inicial oval shaped mesh.
Then there are 3 different materials, one for the outside shape with is the opal glass, and one for each inside geometry micro cristals. That allows to play with the colors of each and easily create
different looking and colored opals. Geometry can also easily be varied to create different optical effects.