The diamond's value was significantly reduced by a tiny flaw visible under magnification.

flaw 是名词，意思是"瑕疵、缺陷"，指物体上不完美的部分或损伤。在珠宝领域特指宝石内部或表面的微小缺陷，这些缺陷会影响宝石的价值和美观。