Why are my eyes purple?

I am a 14 year old female, and my eyes are a really deep purple colour. they have been this way since I was about a year old, at first they were blue. I'm 5'9, 128 lbs, I have black hair, and I'm Caucasion. I was wondering if there was actually some sort of genetic sequence that could actually cause me to have purple eyes. My parents both have blue eyes, but apparantly my great grandmother had sort of indigo coloured eyes, like a purpley-blue. I would really appreciate the help, thanks! :)