Topaz

It sits inside stone with yellow-orange crystals poking out of the surface, anywhere from the underground down through the cavern layer. Break the block and the gem is yours. Occasionally you will fall into a gem cave, either a topaz-only pocket or a mixed one holding every gem but amber, and a single trip through one of those settles your supply for the rest of a playthrough. Geodes are the other dependable source.

Feeding silt, slush and desert fossil into an Extractinator throws gems out as a byproduct of processing junk you already hauled home, which is how most people end up with far more than they ever chipped out by hand. Beyond that, a Topaz Gemcorn planted on underground stone grows into a gem tree, so chopping gives you a renewable supply in your own base without any caving at all.

Topaz sits near the bottom of the gem value ladder, so selling it is not the plan. It covers the usual hook and staff line every gem gets, both of which you outgrow quickly, plus the Sandgun if you want a genuinely odd early ranged weapon. The one thing worth going out of your way for is Magiluminescence, a movement accessory good enough to hold a slot for a long stretch of a run. Much later it goes into the Rainbow Phaseblade.

Gems stick directly onto blocks, which is the cheap way to make a plain wall look like a vein. A Heavy Assembler in a graveyard converts them into gem stone blocks for building in bulk, and the Rubblemaker turns them into scattered gem clutter for detail work. That is a surprising amount of decorative range out of something you were about to vendor.

Real topaz turns up blue, pink, green and colorless at least as often as it does yellow. The game picked one shade and committed to it.