Chakra: Can be used for any but extremely helpful with the Crown Chakra
 
Rainbows in crystals are known to promote happy thoughts and positive energy. 
 
Non-Physical Uses
Amplifies effects of other crystals
Amplifies any intention or energy
Can be used to hold any intention
Strengthens and grows aura
Protects against negative energies
Helps you connect to your higher self 
Aids spiritual awakening
Provides clarity
Balances the emotions 
 
Physical Uses: 
Burns
Insomnia 
Vitality
Immune system