Glowforges can’t yet recognize any material except Proofgrade materials with the QR stickers still on them. In the future, they might be able to recognize a very limited list of mass-produced items, such as Mac products, but that feature hasn’t been released yet. It likely won’t ever automatically recognize things like eggshells or glass, because they’re just too similar to many other surfaces. However, you can still engrave such items just fine by measuring their height with calipers and putting the appropriate settings (height, power, speed, lines per inch) into the app. Also, CO2 lasers just can’t really engrave any kind of metal, but they can mark the surface of it.