Gemstone

Early on, this stone appears to be the source of Will's power, but later we see that it is more of a conduit or tool to help him draw upon a deeper source of magic.

Jon describes this as “no bigger than his thumb; smooth and round in a way that there was a small scoop missing in the bottom center with specks of jet black speckled over the mostly turquoise surface,” but Marcus has always imagined this as an orange colored, translucent gem in a more traditional round brilliant cut.

https://www.thespruce.com/turquoise-meaning-purity-and-happiness-1274380 Turquoise is said to be associated with the “Throat, third eye Chakra”.

…

https://trulyexperiences.com/blog/carnelian-gemstone/ Carnelian appears to be an orange stone with symbolism and magic uses dating at least as far back as the Egyptians. That site also says that carnelian begins with a female energy, but as it's exposed to heat, it turns more red as the iron oxidizes and takes on a more masculine energy. This might jive better with the flow of the story.

“”

Try not to overuse the Gemstone Motif