Premo, Kato liquid polyclay, iridescent flakes.
Here's my process for the necklace on the left: I baked a bunch little clumps of iridescent flakes mixed with Kato liquid polyclay on a piece of glass so they were easy to remove. Then I used some cream colored Premo and stuck the 'opals' all over (rough side up) and textured the surface. After I baked them, I mixed some liquid polyclay with a small amount of burnt umber oil paint and wiped the mixture into the rough spots around the opals for a little color variation and then I gave it a final bake.
The one on the right was basically the same process except the opal is shiny side up and I gave it a purple backing for more color. I was also careful to arrange and layer the iridescent flakes so they were laying as flat as possible for more shine and depth.