Amber

The rest of the gems sit embedded in cavern stone. This one mostly does not. It comes out of an extractinator when you feed it silt, slush or desert fossil, and it shows up in the underground desert rather than the general caves, so most players end up with a pile of it as a side effect of a silt run instead of by looking for it. Geodes down near lava level are the other reliable source. If you would rather stop hauling sand around, plant a gemcorn and let a tree grow it for you.

Outside of the handful of amber-specific pieces, it mostly ends up as gemspark blocks and decoration. A few recipes that look like they should call for it use topaz instead, the Sandgun being the one people notice. That is a leftover of ordering: the gun existed in the game before amber did, and it was never revisited.

Real amber is hardened tree resin, not a mineral. That makes it organic and far younger than any of the actual gemstones it sits next to in your storage.