Suddenly being surrounded by all of the mined diamonds from Nidal doesn't actually mean being buried, there simply isn't that much diamond in Nidal when spellcasters are always using it up. Why leave a diamond lying around just being a diamond forever when you could turn it into power for yourself? Diamond doesn't accumulate over time, quite the opposite.
Nidal's current stocks, combat stores, private stashes, combat reserve stores, emergency reserves, emergency private stashes, final emergency reserves, and all other diamonds in Nidal, don't actually end up massing all that much; a 25,000gp diamond is not a large object by the standards of anything except diamonds.
Broom is nonetheless mildly startled by how many diamonds just fell onto the table in front of him while he was eating lunch. He wasn't really expecting any diamonds at all, let alone that many.