In Spectrobes, Cyrus agrees to give Rallen the blue Keystone in exchange for a very rare Diamond mineral; according to Cyrus, legend states that there is a Diamond at the top of Table Top Mountain on Daichi. The mineral can be excavated after defeating the Krawl on the top of the mountains.
In Spectrobes: Beyond the Portals, Diamonds can be found on any planet and not just Daichi. They can be fed to Spectrobes in the incubator, though they do not have any Property affiliated with them. Diamonds can also be sold for 7,500 gura. At one point in the game, Cyrus asks Rallen to get him another Diamond mineral, though Rallen is able to talk him out of it.