One popular abrasive is aluminum oxide, aluminum rust. The surface of any aluminum you have ever seen (unless you've been in some very specialized laboratories) is covered with aluminum oxide. It's a myth that aluminum doesn't rust. It rusts virtually instantly on contact with air. It's just that aluminum rust, aluminum oxide, is different than the rust you see on iron and steel. While the rust you see on iron and steel is weak, powdery, and redish-brown, aluminum rust is tough, solid, and crystal clear. Aluminum oxide forms such a tight crystal structure that once a layer only a few molecules thick forms, it seals the surface airtight so no more air can get in and the rusting process stops. In fact, there is another name for Aluminum Oxide, and that name is sapphire. Sapphire gemstones are just big pieces of aluminum rust.
What I'm wondering is if maybe the aluminum oxide layer on the foil acts and an abrasive and there might be some sharpening effect.