This block is a very rare block, it only spawns in groups of 1 and is found only near the very bottom of the map. It is worth $1000 making it the most valuable block in the game, it is recommended to use the prospectors tool to find this as it can be quite difficult to find.
More information on what this is used for is found here.
Find 8 Diamonds, clear them all out then as a miner, mine all of them and bring them back up to the surface. You would earn $8000 for your team in one trip, possibly winning the game in one.
The chances of the previously stated happening are less than slim in multiplayer due to other players finding the exposed ore before you can get it. If you were in single player then there would not be any need to earn money for your team making this a achievement rather than a gameplay strategy.