What can make a diamond look blue beside fluorescence?

[Joane]

Hi there! I have a small Canadian diamond in a ring. It is .25ct, F VS1, GIA cert, with no fluorescence. It usually looks white but sometimes in the sunlight it looks blue or even bright turquoise. I''m just wondering what else besides fluorescence can make a diamond look blue?

 

[belle]

a clear blue sky will make a stone look blue.