Think about it. Every Gem may have their gem a different color or in a different place on their body, however, what I find interesting is it's usually the same size and shape.
The Diamonds are not though.
Pink and White Diamond actually look like a genuine Diamond. Pink's upside down and White's right side up.
However Blue and Yellow's look cut.
Blue Diamond
White Diamond's Gem, Pink's is the same but Upside down.
Pink Diamond
Yellow Diamond