Silver Necklace w/Stone Pendant

Not sure what the stone is, although it looks to be almost like petrified wood or some stone of sorts. Comes with a unique silver S chain (not sure if it’s 925 sterling, can’t find any markings), but overall the necklace is in great quality. Fairy light weight & only wear is the tarnishing at the clasp. Fits like an 18-20in chain.