Amethyst

Amethyst shows up as purple crystals set into the stone of the Underground and Cavern layers, and breaking that block is the ordinary way to get it. It also falls out of an Extractinator fed silt, slush or Desert Fossil, which is where most of your bulk supply comes from since you are already running those through. Geodes crack open into it, gem trees grown from an Amethyst Gemcorn can be cut down for more, and occasionally a world generates a gem cave packed with the stuff, either an amethyst-only one or a mixed cave holding every gem except Amber.

It is the least valuable gem in the game, which is exactly what makes it the right one to experiment with. Everything it makes is early: the Amethyst Hook, the Amethyst Staff, the bottom of the phaseblade line, and none of it stays relevant for long. What that is worth in practice is learning gem tree placement and growth on amethyst gemcorns before you risk a Diamond on the same patch of ground.

The stacks pile up and clearing them out feels harmless, but the Rainbow Phaseblade wants one of every gem, and walking back down to the caverns for the cheap one because you sold the lot is an irritating trip. Keep a handful in a chest. Past that, amethyst can be placed on blocks as decoration, turned back into gem stone blocks at a heavy assembler inside a Graveyard, and used as purple rubble when you are building.