The gems' rooms are based on how they were made.

Amethyst's-Amethyst is a type of Quartz and it grows in messy, unorganized clusters. Hence the garbage piles.

Pearl's-Pearls are grown in water. Hence Pearl's room having a lot of water inside.

Ruby's and Sapphire's-Yet to be seen.

Rose's-Room that fulfills desires.

If Rose is Rose Quartz why wouldn't her room be like Amethyst's, a fellow Quartz? Rose's room should be messy and unorganized but instead it asks for what Steven wants.

Diamonds are said to fulfill desires. They are supposed to make you less shy and release your hidden desires. More proof of Rose being a Diamond.