Seriously, do you ever look at someone and think: her eyes look like amethysts?
It's not too bad if it's an inhuman character but if you add in the purple prose description...It's also another reason to hate elves. People love giving elves 'sapphire' and 'ruby' eyes.
I've been in love, but I've never looked into a boy's eyes and thought they were sparkly like jewels. Maybe murky, or in-between two colours, or I might notice that tiny bit of lighter colour around the edge that makes them look brighter, but this is far from the realm of jewels.
They may be the windows to the soul, and we naturally seem to have grown fond of talking about them...but please, spare me the precious eye-jewels, and the ones that go crimson with rage.