There's not much to say on this topic but here are a few tips to remember.
The Gem's skin and clothing always should match the color of the gemstone.
If the gemstone has any unique physical features, (ex. 'cracks', patterns, whatever else) use them in your Gem's appearance.
IDK what else to tell you, just make it PRETTY.
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How to Make A Steven Universe OC
De TodoI've been more interested in Steven Universe lately, so I thought I might make a book to help others in the fandom make a Gem OC. :^> EDIT 7/15/16: I don't plan on updating anytime soon. There may be some inaccuracies in this guide so be wary :0