Can someone just... explain netherstones to me?

Maybe I don’t understand them correctly, but I am currently on realm depth 155, but have not found any netherstones since finishing the main story. Do they only start appearing after a certain realm depth? After beating all the nether bosses? I know they have a pity counter, but with how that works, I think I should have at least found one at this point.

But it could be that I’m just misinformed, so I’d gladly just have someone explain them to me again.

They’re very rare and can only be found on your highest-ever realm depth. You’ll start to find them more often as you unlock more blessings, increase realm instability/depth, and generally improve the amount of loot you receive.