Eli5: How do jewelers cut gems evenly and precisely?

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How are there always the right amount of sides, of the perfect size, and all these different little faces and stuff? I know they basically use a big sanding wheel but how do they not have one side bigger or smaller than the others?

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From what I’ve seen my brother do, a lot of math and a lot of practice. He does more cabochons than gem cuts but as far as I know they just map it out (marking on the stone) and then cut accordingly. Often, especially on the smaller ones (most of the ones you’d find on rings) they use lasers, but I haven’t actually ever seen that so I have even less an idea how that works.

I’m in Diamond manufacturing. We use lasers that scan diamond rough and tell us precisely what to do.

Yes they use a big sanding wheel like you said.

They end up with the sizes exactly right because… they’re very good. I don’t know what to say. Your question is like asking how builders make buildings that aren’t crooked or how watch makers assemble tiny mechanisms that keep time for years. They have the right tools, training, and have practiced a very specific skill for hundreds or thousands of hours.

Someone in the gem cutting profession is called a [lapidary](https://en.wikipedia.org/wiki/Lapidary), and the good ones are master craftsmen. If you were looking for some secret info, sorry. They’re “just” tradespeople good at their trade. In a way, I think that makes it even more impressive.