THCA crystals are the crystallised acid form — the shape the compound naturally takes in the plant until it is heated.
They form through controlled crystallisation: a solution is left to evaporate slowly and crystals grow out of it over time. It takes days, and the rest of the solution stays around them as a liquid phase.
The crystals themselves have no smell and no taste — everything aromatic stayed in the liquid. Which is exactly why they are sold together with it, as Sauce.
They put one extra demand on the glass: a crystal vaporises more slowly than a soft concentrate, so it needs a steadier temperature. A thermal banger and a carb cap do more work here than a thin-walled bucket.