[@Raijinslayer]

My theory is all base forms that appear are technically made to look like one of these(see pic down below). It isn't a human form, it is just the model that the base form seems like human.

[img]http://www.artsuppliesdirect.com/images/products/detail/12inuni.jpg[/img]

The glitch or corruption can't change a base form directly, no matter what they do so they places the corruption in the details so that it would change the base form of the newly made gem. It makes them unable to be changed by healing, they are made to not think straight because my guess is they were hoping that when they came out they would listen to orders with no question. Many "normal" gems chose Rose which means that maybe the diamonds didn't fully trust their own to follow orders and fall in line like they always had. 
That is way they call anyone like Rose defective.