My theory on this is that the fusion doesn't contain Amethyst. The gems that have Amethyst keep her gems shape. A circle. While the temple has an oval instead. The Centipeedle's gem has an oval in the center which would be the only part you could see of the gem. It's also the heart chakra, so it would be in the chest. This would make sense considering that Rubies, Sapphires, and Emeralds, are considered precious gemstones as well as Diamonds.