Let me try to save you some time and aggravation with your test sample.
Your epoxy granite mix requires essentially three things:
1. A "dust mix" ranging from #36 to sub micron,
2. A "large mix" ranging from 1mm to 6mm.
3. Epoxy binder.
For the dust mix, use various sizes of Aluminum Oxide industrial abrasive. The kind used in sandblasting. It's easy to find in #36, #180, #220 and #320 sizes. Local blasting companies might even have some laying around from previous jobs. Mix even amounts of each size. 50/50, 33/33/33, 25/25/25/25, you get the idea.
For the large mix, buy a bag of washed pea gravel. The sizes will be in the range you need.
Make the dust mix about 14-15% by weight of the total aggregate amount.
Make the epoxy (with hardener) about 10-12% by weight of the total weight of the piece.
For example, if the total piece with epoxy will weigh 5000g., the dust mix should weigh about 640g, the large mix should weigh 3760g, and the epoxy (with hardener) should weigh 600g (2:1 would be 400g/200g).
Try it out in a baking pan, it's easier to use for testing.