oil-spot
/ˈɔɪlspɒt/noun
1- an oily patch or mark油点。
1.1
- a silvery marking on brown Chinese porcelain (especially of the Song period) caused by precipitation of iron in firing（尤指宋朝时期褐色中国瓷器上由于烘烤时铁的沉淀而形成的）银色斑纹。