Opal is another one of my favorite stones and there are several methods to mimic this sparkly gem, but I like Donna Kato's tutorial the best. Here are a few of my favorite pieces I did while experimenting with this technique.
The mica flakes appears different each time, depending on the colored clay you use for a background. The heart on the left for instance is with a plum colored clay where the one on the right was done with black. The dragonfly wings below were with a silver background.
Right now I'm experimenting further with this technique but using inks and foils as I'm trying for an effect more similar to the Mexican opal like in these earring pieces below, so we will see what happens.