Pitchblende is a magma-safe purple stone which occurs in small quantities within granite.
Pitchblende is also especially heavy, second only to cinnabar (and slade), so it is absolutely inappropriate for making large pots and other commonly-hauled items, and preferable for stone-fall traps and catapults. Additionally, pitchblende crafts are so heavyweight they can fill up a caravan. Hauling pitchblende is not realistic, or at least will take an eternity, without a wheelbarrow.
 In Real Life
Pitchblende is an ore of uranium; while it cannot be smelted in Dwarf Fortress, it is often used for decorative purposes within "nuclear reactors" in megaprojects.
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It has been speculated that should the Dwarves work out how to refine and enrich uranium that the world would face a super weapon even greater than that of the !!Thermonuclear catsplosion!!, as well as an incredible new power source. It is believed that Dwarves would proceed to use the new super-weapon to preemptively dispatch invaders, and ignore the potential new power source (since they have worked out how to create perpetual motion machines). Or they would find some way to brew it, giving them all mutant superpowers after a night of drunken partying.
this is uranium oxide, but was possibly used for glazing ceramics (colored glass most likely came later involving refinement beyond our tech level)