poem poetry jewel jewellery 用法 类似于谓语单复数之类的

1.可数：poem,jewel,pot,
2.不可数：poetry,jewellery,pottery,
正确：a poem/five poems
错误：a poetry/five poetries可以说 a piece of poetry 吗？Absolutely yes, I myself have done so. However, more often than not we would rather use "a poem" than "a piece of poetry".