Today’s dental patients have two options for tooth-colored crowns: metal-based and all-ceramic. Porcelain-fused-to-metal (PFM) crowns have a cast metal alloy core with a porcelain covering. While sturdy and safe, PFM crowns can cause a grey line to form between your gums and teeth as your gums recede. All-ceramic crowns don’t have this problem because they contain no metal at all. Porcelain materials allows light to pass through, mimicking the translucency of real teeth. On the other hand, metal blocks light, making the restoration appear less life-like.