It's a Byzantine Empire coin, Constantine VII Romanus I, made around 921-944. Believe it or not they are fairly common in Gold, a bit more scarce in silver though. If it weighs around 9 grams and it is silver it might be real but the fact that it was used as a pendant makes it doubtful. In Turkey they sell many replicas on the streets like that. If real anywhere from $25 to $150.
