All amber is fossilized tree resin. Baltic amber in particular (mined from near the Baltic Sea) additionally contains a substance called succinic acid. When worn on the skin, body heat causes the amber to release an oil containing succinic acid, which when absorbed into the skin, works like a natural Ibuprofen, blocking pain and reducing inflammation. Note that succinic acid is an alkalizing acid (like lemon juice) so it won't add to your body's acidity, and may actually slightly lower it.