Sturdy Fossil

The reliable route is bulk. Desert Fossil is everywhere in the Underground Desert and quick to mine, and only a slice of what you feed an Extractinator comes back as the sturdy kind. That ratio means you want a full inventory of raw fossil per trip, not a stack or two. Crate fishing in desert and oasis water adds a slow trickle, and the big worms and lizards down there drop it outright.

Kitting out the whole fossil line takes a serious pile, so decide what you want before you start spending. The armor and the pickaxe are the parts that matter for an early run. The throwing gear eats fossils fast for damage you will outgrow within an hour.

The block itself is worth nothing at a shop, so a chest of spares is dead weight. Craft the surplus into Bone Throwing Knives and sell the knives, which is the only conversion that turns it into coins at all. As a building material it blends with nothing but the desert fossil it came from, and a Metal Detector will flag it even in walls you placed yourself, which gets noisy in a base built out of the stuff.