Hello my friend, I know black onyx diamond rings in this exquisite style were at the height of popularity during the mid 1800s through 1930s, so there is a good chance the original diamond is a real. Since cubic zirconia (CZ) was not officially introduced into the retail market in the US until 1976 (see reference website below), I would bet that the original diamond was real and would recommend taking the ring to a pawn shop to have the diamond checked as they have nothing to lose if it is real - whereas a "fine" jewelry store has all to gain by suggesting it is fake and having you purchase/replace the gemstone by them. The "k" would most likely indicate the karat of gold and would also indicate it was made in the US, making the ring's age anywhere between 1910 and 1930 (Art Nouveau to Art Deco). Any other markings could help you identify who made it.
