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It's worth bearing in mind that every type of gemstone is given a rating of hardness on the mohs scale, and if you plan to wear your ring often then you'll ideally want a gemstone with a rating of 8 as a minimum, ideally 9 or 10. Certainly 9 or 10 if you plan to wear it every day.
You can get away with softer gemstones if you're prepared to baby them, but do some research on how your chosen gemstone is know to degrade (some have certain ways in which they shatter or cleave in two) so you know how to look after it. Alternatively you can have a bezel setting but it will still be open to scratches and chips.
Diamond's score of 10 is part of the reason it became so popular as an engagement ring stone.
Here's a link with the scale x
http://www.gemselect.com/gem-info/gem-hardness-info.php
I would consider a sapphire personally. You can get one in a beautiful pink or green, and they'll have the durability you'll need to wear it as an e-ring.