Measuring Gemstones?

I was curious about measuring gemstones, like amethyst. I know all about diamonds, and their deal, like with what you could consider a large diamond. But I know nothing about gemstones! My husband bought me a 8mm amethyst pendant for our 1 year anniversary, and I was wondering if that is big for a gemstone, because when we picked out the necklace it looked huge to me lol, but I was thinking that may be because I am use to diamonds. Can anyone explain more about this to me, and tell me if 8mm is big, little or normal?