The Diamond Spider was rare creature to find in the universe. But, when the Doctor, Amy and Rory arrived on a spaceship, they realised it was caught on one of the spider's Diamond webs.
It produced diamonds in its body naturally just be eating and digesting metals including ships and asteroids it has caught.
Deciding it was valuable and would earn them a lot of profit, The Empire Of Eternal Victory attacked it in order to capture it and use it to create diamonds for themselves so that they could be rich. Instead, the Doctor stopped their plan and the spider was allowed to be free. (NSA: Web in Space! (novel)