Alexandrite is a beautiful bluey-purple gemstone (though it can vary from green to red as well) and is variety of chrysoberyl. At 8.5 on the Mohs scale it makes a really durable choice for an engagement ring. Becoming more stylish in recent years, it can be found in Russia, India, Brazil and Tanzania, among other places, but also look out for the more affordable, but equally pretty lab-grown varieties.