system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do jewelers decide what shape, angle, # of facets, etc. is going to work best with a particular gemstone?

Do certain types of gems have standard cuts, which work because of the internal reflection / refraction of that substance? How are new cuts invented?
gpt
Well. Which cut a gemcutter selects depends on the clarity and impurities of the gem as well as the shape of the raw gem (ideally a gemcutter wants to retain as much weight as possible while still enhancing the attractive features). And also fashion. For example when it comes to diamonds the emerald cut (creating a rectangular step cut stone) was popular in the 1930s, while in the 80s the brilliant cut was the most favored because it enhances the glittery aspect of the stone. Today the brilliant is still one of the more favoured cuts, but the rose cut is making a comeback because it gives a rock a more old-timey feel.

Generally if a stone has a high clarity the gemcutter will want to pick a cut that maximizes the brilliance and a fairly shallow cut (because that creates the maximum light interplay) while a with a "fancy" (a rock with a lot of colour impurities) the gemcutter will want to enhance the colouration and will pick a shape suited to the nature of each gem.

With a rock that's lower quality the gemcutter will probably favor a deep cut to retain the maximum weight, since even with a shallower cut the light brilliance won't be all that spectacular.