Wiki says it's given to you by an Elite Trainer on Route 14, but there aren't any on that route from what I can tell, unless I'm missing something obvious in the diving portion of the route.

Aside from that one, my mega stone hunt is pretty much done. The only other ones I haven't gotten currently are S51-A's UFO and mystery second mega stone, Dramsamite, and Daikatuna's mega stone, which I assume are all either unreleased or only obtainable through the 7th gym for 1.0.0.

EDIT: Got Syrentideite. Thanks for the help on that end. Now, is there confirmation somewhere that the rest of the mega stones are unavailable or only obtainable through the 7th gym as of 1.0.0?