noun
unpolished gem; uncut gem; rough gemstone
Refers to a gemstone in its natural, unworked state, before cutting or polishing. Often used metaphorically for something with raw potential.
彼の才能はまだ粗玉だが、磨けば光るだろう。
His talent is still an unpolished gem, but it will shine if refined.
この原石は粗玉の状態で美しい。
This rough stone is beautiful in its uncut state.
Compound of 粗 (ara, 'rough, crude') and 玉 (tama, 'ball, gem'). The exact historical derivation is uncertain, but the word transparently means 'rough gem'.