Ruby

Red gems sit inside otherwise ordinary stone throughout the underground and cavern layers, and they are easy to walk past when torchlight is doing all the work. A Spelunker Potion is the difference between finding some and finding all of them on the same trip. Occasionally a world generates a pocket packed with nothing but ruby, or a mixed chamber holding every gem except amber, and one of those covers every recipe you will ever want in a single visit.

Silt, slush and desert fossil all give up gems through an Extractinator, which quietly turns the junk you already haul out of a mine into steady income. Geodes in the underground desert crack into gems as well and are worth breaking on sight. The permanent answer is a Ruby Gemcorn: plant one in the cavern layer, wait, and you have a tree you can fell and regrow forever. That is the point where the gem stops being something you ration.

Most of it goes into crafting, and red is the color you burn through fastest once gemspark blocks enter the picture, because lighting an entire build with them takes far more than a casual pile. The Rainbow Phaseblade wants a handful, which is a very different scale of demand. Rubies also run backward: a Ruby Stone Block at a Heavy Assembler inside a Graveyard turns the gem into a building material, and the Rubblemaker scatters loose gem clutter if a room needs texture rather than light.

They stick straight onto a wall like any other gem, and a placed one sparkles on its own without a light source behind it. In a display room or a trophy hall that reads far better than another torch, and it costs nothing but the gem. Worth holding a small stack back before you feed everything into a recipe.