Location of the diamond rock on the interactive map: here

Loot spawned by lifting up rocks does NOT regenerate with the Blood Moon - that’s a common misconception. Instead it respawns via the “RevivalRandom” gameplay mechanic, which is a bit more complicated:

• Loot from that category has a 1% chance to respawn for every minute of real, active playtime that has passed. Time spent in menus, cutscenes, loading screens etc. doesn’t count, and the amount of in-game time that has passed doesn’t matter. You can not just put the console in sleep mode or skip a few days at a campfire to make the loot respawn faster - only actually playing the game will make that “timer” run.

• In addition to that, it also only works if the player is not on the same map grid during that time, so stay far, far away from that mountain. It won’t work otherwise.

Since the loot respawn is RANDOM, you’re also not guaranteed to get that diamond to spawn in a specific time frame. You might get very, very lucky and it’s back again in a mere ten minutes, but you might as well be very, very unlucky and it did not respawn after 20 hours. It will respawn eventually, but the time frame may vary.

It takes an average of 3-4 hours for a 90% probability.

If you want a faster method to get diamonds (for armor upgrades or reforging the Champion’s Weapons) then you can also trade 10 Luminous Stones to Ledo in Zora’s Domain. Just don’t do this “for cash” as the 10x Luminous Stones are worth more than the diamond (200 rupees difference, to be precise). Or you can go on a Talus / golden enemy killing spree and hope for the best.