Decoden (デコ電) is Japenese and is short for decoration denwa, and denwa means phone. It is when you decroate your phone with lots and lots of jewels and you can even make a 3D designout of jewels.
From kawaii.com modes4u.com and full moon.com
You can make your own! All you have to do is mix flour (any kind) with any color of acrylic paint. mix it until it gets to be like frosting and there you go! practically free decoden!
I would suggest any air dry clay because with polymer clay you would need to bake it and use tls instead of glue. You will need to wait longer for your decoden piece to dry but it's much safer than putting the thing you are goin to decoden in the oven.