You are more likely to spot the boss alter from afar than find the rare location stone.

Yag is stupidly tough to find on some maps.
The alter looks like a giant (read, size of the stone columns in the plains) skeleton hand coming out of the dirt, so roughly, 'fingers' of stone columns. If you see more than 2 stone columns right on top of each other, its probably it.

It seems to frequently spawn in sight of shore, if you want to boat around. But my sample size is tiny, as I have 3, maybe 4 serious maps.