So I was on my server today playing around and I got swarmed by zombies near my house, no problem there. Except when I tried to go grab my gravestone, i was unharvestable. Other things I tried to harvest around me worked fine, only the grave was unpickable. This proved very confusing to me. It eventually dawned on me what it could be, and sure enough, I had somehow ended up in Adventure mode rather than survival, from having been changing gamemodes to test things.

I figure adventure mode for the most part doesn't want you harvesting tile entities or something, but I think making the gravestone an exception if possible would be wise, as the block serves no real function in the survival gamemode if it's uninteractable, whereas it'd be pleasant for use in custom maps otherwise. The shovel use didn't work either. I thought it was Iguana's Tinker Tweaks messing with me haha.

@boq Chisel 2, Blood Magic, ... have blocks that can be removed by pickaxe. I just tried this in the Direwolf 20 modpack

Well, last week I noticed some code that looks relevant, so I may try to do something with this again.

EDIT: well, false hope. Apparently pickaxe works on every block declared as "rock" material, but spade works only on snow (while graves are "dirt" material). I may consider changing grave to "rock", but it has some weird side-effects.

After quick look at code, it seems that list of blocks that can be removed by tools in adventure mode is hardcoded, If you know any mod that contains block, that actually can be digged in that mode, please let me know.

Anyway, I recently added feature to graves to drop all items (without destroying block) when clicked with shovel.

And of course, you can disable graves per world with gamerule openblocks:spawn_graves.