“天然钻石”常见的英文表达是 “natural diamond” 。
例如：
Natural diamonds are formed deep within the Earth over millions of years.（天然钻石是在地球深处经过数百万年形成的。）
She prefers to buy jewelry made with natural diamonds rather than synthetic ones.（她更喜欢购买用天然钻石而非人造钻石制成的珠宝。 ）