Emerald

Green gems sit in the walls of the Underground and Cavern layers, inside blocks that visibly sparkle, and chipping them out one cluster at a time is the slow route. An Extractinator is faster in practice, because it hands you gems as a side effect of processing the Silt, Slush and Desert Fossil you were already shoveling. Geodes in the marble caves crack open for a decent handful each and are worth breaking on sight.

The jackpot is a gem cave. Some worlds generate a pocket packed with a single gem type, and others generate one holding every gem except Amber. Either one turns a long running shortage into a non-issue in a single trip, so it is worth spelunking wide before you commit to farming them any other way.

The permanent fix is a stand of Emerald Gemcorns planted on stone underground. Gem trees regrow after you cut them, so a small grove retires the mining problem completely. Start one the first time you have acorns and a gem to spare, because the entry cost is a single emerald and the return has no ceiling.

Once the weapons and hooks are covered, emeralds are decor. They can be stuck straight onto blocks as sparkling green studs, packed back into Emerald Stone Block at a graveyard workstation, or handed to the Rubblemaker for scatterable gem clutter. Builders burn through them at a rate no fighter ever will, so do not clear out a stack just because you finished the Rainbow Phaseblade.