Amber, not to be confused with my sister's name, is basically fossilized tree resin that's usually 30-90 million years ago. It's not just insects, like this unfortunate mosquito illustrated here that get trapped in the resin prior to fossilization, plant remains, spiders, frogs and even lizards can be trapped. Though not mineralized, people have been widely using it as jewelry for centuries. Resin is produced by trees as a way of healing themselves from biting insects and disease.