Diamond

Diamond is a costly gem. It can be found in small quantities on any planet (even on the first Garden world, but very deep, in the core layer) or sifted from Ash Pile or Volcanic Ash.

It's extremely valuable in the early/mid game, because 15 Diamonds can be traded for a Teleporter Core, and you need those for Item Transference Devices (a cornerstone of automating your base).

Precision Ore Detector (when upgraded to tier 2) is very helpful for finding Diamonds.

Once the player obtains Blast Furnace (which requires all tier 3 ores), Diamonds can be produced by smelting Carbon.