Obviously not based, as i do that in batches when i have a dozen or so models to do. Im only posting this because a few people showed interest when i mentioned Diorite.
I took an overly watered down black paint, saturated a brush, and slapped it against a toothpick to get the black dots all over the stone. Dont think i did that quite right, since they all ended up either really difficult to see or i had to wipe it off because it made a giant spot.

I do like the central gem idea far better than colored streaks all over the golem, however i dont think it looks very gem-like atm. Anyone got any pointers on that?

Try base coating the gem silver and then coat it with a blue glaze. Saw that on a WarhammerTV video on youtube. Might be worth a try.

Gems are transparent. So what that means for you, is that although the side with the catch light had one (which you have), most of the color and brightness otherwise is on the opposite side. So you should bring up the colours on the bottom, slowly, with several layers.