Though you have a great point, my only personal concern with this theory is the fact that no matter how good a gem is at shapeshifting, they cannot fully replace their gem. When we saw Amethyst shapeshift into Jasper, she appeared to have Jasper's nose gem, but didn't change her own gem, simply covering it. Other wise, a gem as powerful as a diamond should be able to hold that form for longer than our Amethyst.