Black Onyx (?) and Marcasite Heart Necklace

Pendant and bale is marcasite with heart shaped black onyx (?) stone. Pendant and chain are sterling silver. Chain marked Italy & 925. Heart pendant is 6 grams and chain is 3 grams. See photos for approximate measurements. Thank you for your time to browse my closet! I really appreciate it! Always glad to answer questions. Smoke free and dog loving home. Inv# Bin