The title basically says it all lol. Is it from a specific boss that drops it? Or is it just from luck?

Stonewear gear is from the lost grove if Im remembering right. Don't remember the details but it's a lost grove item(s) and the reason the blacksmith is a thing.

Ooh! Ive been wondering what the blacksmith was for. Thank you for clearing that up!

The blacksmith is for upgrading lg equipment (all lg equips basically start as "+0" and can go up to +5), for making the blueprint avatar items from lg elites/bosses, and for creating stoneweave gear.

Stoneweaves require Ancient Tomes (might not be the right name, generally just referred to as 'Tomes'), a legendary drop from the Cave Bosses in the LG (Dia, Kowlla, Demora).
One Tome allows you to unlock ONE kind of Stoneweave (you need a total of 18 tomes to unlock them all, as it's divided by element and equip type- so Reaver Relics are separate from Pyromancer Relics are separate from Pyromancer Bracers etc.)
Once you unlock the 'blueprint' that blueprint is unlocked forever, but you still need to individually craft each one you make. Enigmatic Gems (legendary boss drop from all lg bosses) are used in upgrading (and maybe creating? I've never actually done it) Stoneweave gear
Hence why Stoneweaves are so expensive

Ohh, thanks!