How to tell him I don't like it without hurting his feelings?

My boyfriend of a year bought me a birthday gift, a necklace with my birthstone in it. I absolutely loathe the peridot. I hate, hate, hate the peridot--I'm a sapphire girl. I know he did it because he cares and he wanted to get me something nice and it was very sweet and I love him for that. I did the appropriate thing and told him I loved it... then he started talking about completing the set for me. The last thing I want is for him to spend boatloads of money on a set of peridot jewelry when I hate the gemstone. How to I tell him gently that the peridot just doesn't do it for me?