All the Diamonds (Murals and Actual Appearances) have Gems that have a face with 4 sides making a diamond. And what's below the what I believe is the Facet, is regardless as those bits can't be seen. The ones we suspect to be Diamonds have either; a circular Facet/Face (idk if facet is the right term but whatever), or seem to be to angular to get a Diamonds Gem. This would be because they would pertrude out of the form, similar to Jaspers Gem whereas a Diamonds lays more flat. Any Gem can look like a Diamond as, in real life we cut Diamonds to be different sizes and shapes. However, in Steven Universe a Gem has a universal shape which they all follow, an example being the Rubies. Whereas Yellow and Blue are variates of the same type and differ slightly but follows the same principle of shape. In terms of Quartz Gems as a side note, Rose and Amethyst are Macro-Crystalline and are more circular, whereas Jasper is Micro-Crystalline and is smaller and shaped differently.
Sorry about my poor formating, I was writing in a mix of my short hand which doesn't use Punctuation. I've removed my previous comment to keep the page cleaner looking :3.