One term that I keep seeing, but I'm unfamiliar with (the term isn't used for "modern" coins) is "horn silver." Could you guys please explain: 1. Exactly what it is? 2. What causes it? 3. What effect it has on coins, especially the surface underneath? 4. How it affects grade and value? 5. Any conservation methods which are effective? And, if you have pictures to show what it is, that would be helpful. Thanks!