It seems like the general meaning is when something is round, or at least rounded – 勾玉 / 火の玉 / 目玉 / 温玉. But 玉 can apparently also mean 'gem' or 'jewel' (especially spherical – but not always?) or 'coin' – so it doesn't seem to strictly refer to spheres as the only viable shape.
So… where's the boundary here? When is something 'round' enough to be considered a 玉 – or do gems and jewels not have to be 'round' at all to qualify?
by ElementsnStuff