Haggy,
I've had no problem with the granules coming out/breaking away, the epoxy holds them very well. Having said that it's a messy job. But I saw a YouTube film the other day of someone setting minerals with CA, so I might try that next.
I've been oiling the vase day and night for a while now, it's really starting to build up a high shine, I'll pop-up a close up of the granules sometime soon so the effect can clearly be seen.
As for silica I'd like to see the effect that creates (hint, hint - piccy please).
I've just started playing with Mica suspended in liquid pigment. So far only as it comes - a Matt effect, the effect is very subtle, you see it without knowing what you see, I'll try to find something to raise it to a gloss sometime. Unfortunately it does not photograph at all!
I imagine Mica would look better using your method, the mica being in a larger 'mass'.