ive been toying around with the idea of making a subtype of the monk who has the ability to use KI to transmute their skin into different elements. 

I know the monk class already gets to use bigger dice for unarmed strike at higher level ...but i feel like this opens the doors to a lot of cool roll playing things ...  thoughts? 

My thoughts are as follows:

• It seems like there are no level caps for the objects needed, I would start with some limitation on that.
• It doesn't say if it consumes the object, or can I buy one diamond and have diamond skin for as long as I have Ki points?
• There is also no explanation of how long the transformation lasts. Does it work like concentration, or have a time limit?
• I also think +5 AC is a bit ridiculous lol I might want to go with +0 AC from Bag Full of Earth (Dirt?) all the way to River Stone. Then +1 to Piece of Refined Iron and +2 to Diamond, as to not completely break the game.
• You would also need to think of weight as a player. Each form should have a multiplier for the characters weight. Character Weight x 1.3 for Bag of Full Earth (I just threw that number out there, I have no idea what the modifier would be) Also, would their Strength be high enough to still be able to move (Speed Reductions) or disadvantage on Dexterity checks, saving throws. Rules of encumbrance?
• This seems like it could be factored into the Monk's Way of the Four Elements, since you are only transmuting into earth-based materials.
• I think this is a VERY groovy concept for sure. However, I think there are more things to think of, especially since you are legitimately transforming your entire body into another matter.

Hope I helped! I look forward to your replies and changes (if any) to this.