well it won't be cheesy if you use it right, and be careful some girls get a bit freaked out when a guy just randomly goes up to her and says she's got diamond eyes. But in general it'll probably seem quite cute to just be engaged into conversation, and then to just say she has diamond eyes after you've been looking into them,... seems kinda sweet but it worked on my mate once. Although the guy came up and said that he couldn't help but come over and talk to her because her eyes had been sparkling all night.
DON'T say something like 'i wish I could sell your eyes because they're like diamonds'
um not sure what else to suggest. hope this helped a little bit x