What does the locket say that Harry found?

The locket Harry found in the Half-Blood Prince's cave did not have any inscription on it. It was a dark, heavy locket, made of silver and with a single, glittering, red stone in the center.

However, the locket was a Horcrux, a magical object containing a fragment of Voldemort's soul. The locket was enchanted with a powerful dark magic that preyed on the wearer's deepest fears and insecurities. It was only after Ron and Hermione destroyed the locket's magical properties that Harry discovered its true nature.