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 is 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.

Automation
Automating diamonds is very useful because of it's many applications. It can be done by pumping certain liquids (most efficiently Aether or Caliginous Gas), centrifuging them into carbon and then smelting it in a Blast Furnace or Arc Smelter.

Many byproducts will come from this process, some of them rather useful. Ash Pile and Pure Carbon can be processed for more diamonds.

I recommend the usage of Industrial Centrifuge as it is cheap, fast and do not consume a lot of electricity. The lower chance of carbon per liquid is offput by it's speed.

Remember to pump the liquids from oceans and not puddles!