I received a response from the PA State Museum.. Very cool!!
I am Steven Jasinski, the Acting Curator of Paleontology and Geology at the State Museum of Pennsylvania. You have actual found a fossil vertebra rather than a pendant. It is part of the tail of a cetacean, a small one, and, to be more exact, probably a species of dolphin. The triangular extension on the top is the pointed part of a spine you feel when you rub down someone’s or something’s back. The small opening is where the spinal cord travels through. The larger part is called the centrum and the somewhat sunburst-like pattern is one of the ways I know it is a vertebra from a mammal. It shows that the vertebra would have had a cap on the end, called an epiphysis. When mammals are young and growing, the ends of the bone don’t fuse together with the rest of the bone so that the bone can continue to grow. When the animal is done growing and has become a full adult, the ends of the bone fuse onto the rest of the bone. So mammal vertebrae have two caps (one on either end), and when the animal is done growing those ends fuse to the rest of the bone. The fossil vertebra you have doesn’t have those end caps, so it is not fully grown. The specimen is probably in the range of 10,000-25,000 years old as sometimes fossils of this age are found in the ocean just off the shore of New Jersey. Yours probably eroded out of the sea floor and washed up on the beach for you to find. So you have found a fossil dolphin tail vertebra. Very interesting find as I’m sure many people would just overlook it. If you would like to know anything else feel free to ask.