diamonds. I hate pearls. (There's a good chance the following remark will mean nothing to anyone on here except for SomeWayOuttaHere, but...) I wonder where SomeWayOuttaHere is. She has a thing for pearls, as far as I know.
As a guy, I wouldn't wear either. But if you're asking which I'd prefer to look at, it would be pearls. They are so much classier than diamonds. I think we're going to see a backlash against diamonds in the coming decades. Part of it is ethics, as human rights seem to be a big problem at many diamond mines, and part of it is the fact that diamonds are just too "glitzy". It's like wearing clothing with flashing lights on it. There's a reason why diamonds are so popular in hip hop videos alongside Lamborghinis, strippers, and guys holding huge wads of hundred dollar bills. They just aren't classy anymore.
Another reason diamonds could lose popularity is they are going to become more and more common. With advances in technology, people are now able to make real diamonds (they're made of carbon, just like us). As that technology develops further, there will come a day when people are able to "print" a diamond bigger and more perfect in color than anything that can be found naturally.
Diamond or pearl earrings are the two most classic pieces for a woman to have in her jewelry collection. They compliment everything you wear, you can wear them everday and they are the building blocks for future jewelery, i.e. pearl necklace, pearl bracelet, etc.