You can gain oneexotic gem for each of these things you destroy. (Much like when you destroy trees, you get growth sap.)
There were up to four small islands together, littered with these gem clusters. It takes approximately two Blast attacks to take down onegem cluster. I wasn’t able to find a pattern in the gem cluster colors that would suggest which gem you get, as it seems to be randomized.
I’ve seen these before; based on personal experience, the only Exotic Gems these clusters can’t drop are Painites and Musgravites, both from Dark Sealed Chests.