"水晶之一种" can be translated as "a type of crystal" or "one kind of crystal" in English.
Here are two example sentences to illustrate the usage:
1、 Amethyst is a type of crystal known for its beautiful purple hue.（紫水晶是一种以其美丽的紫色调而闻名的水晶。）
2、 This mineral is actually one kind of crystal with unique properties.（这种矿物实际上是一种具有独特特性的水晶。）