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.

Diamond can also be found frequently on Chromatic worlds

Automation
Automating diamonds is very useful. 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. You can also use Sulphuric Acid, which doesn't exist as an ocean, but is infinitely mixable anywhere.

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

When using infinite oceans (e.g. on Aether World planet), Industrial Centrifuge will be most efficient. It is cheap, fast and does not consume a lot of electricity. The lower chance of Carbon per liquid is fully compensated by its speed.

If you are beekeeping, Crystalwing bees can produce a Crystalline Honeycomb, which is extracted into Diamonds.