Epoxy clay is a two-part clay that you take equal parts and knead together and make a coloured clay that you can put crystals in. And other stuff. It is strong ? very! - waterproof, and super sticky. Once you stick it to something, after it has cured, it's there.
You have about 30-60 mins of working time, before it is too stiff and loses it's stickiness. After that, it needs about 24 hours to cure, and it cures faster in a warm room.
Here's some Inspirations to give you some ideas of how to use it!