Sometimes it's called a peyton, in this case I'm talking about a flat platelike disc with a pentagram inscribed on it. Sometimes they're metal and sometimes wooden or even wax (left over from the days when having a pentacle carved anywhere could cause you to get burned; a wax pentacle could be melted and destroyed quickly). They're symbolic of the element of Earth, aligned with the Goddess and the north. This is the thing that you place in the center of the altar when you're doing a ritual. A lot of the time a ritual will focus on a particular symbol, and that's what you put on the pentacle. You can also put things on it to consecrate them, and you can use it for anything from placing symbolic candles on to using it as your plate to cut a symbolic apple. It's kind of a rip-off to buy one; they're REALLY expensive even for a small one. You're better off doing what I did: make your own. (Mine used to be a circular wooden cutting board; now it's a ritual object! It's magick!)

Magickal tools