Almost all minerals are crystalline (part of the definition of a mineral), but only under special conditions do you form minerals with well expressed crystal form(s). The mineral may grow from a melt as in igneous rocks, from solids under elevated temperature and pressure, or in open crevices or pockets from hot, warm, or cold mineral-laden fluids.
So, when you examine euhedral crystals of any mineral, please realize that they are rather remarkable objects. Probably 99 % of all minerals present in the earth's crust display no external crystalline form. A euhedral crystal is a flower of the mineral world! Treat crystals with care and respect because they probably survived millions of years before someone discovered them. One careless moment and they may be destroyed forever.