Complex sentence: The jewel is a gold diamond-shaped pendant with a blue sapphire at the top dating to around 1460 that is engraved with a picture of the Christian Trinity on the front and of the Nativity of Jesus on the back.
Split into simpler sentences: The jewel is a gold, diamond-shaped pendant with a blue sapphire at the top. The pendant, dating from c.1460, is engraved with a picture of the Christian Trinity on the front, and one of the Nativity of Jesus on the back.