Which choice shows the proper way to join two independent clauses with a semicolon?

A: Michelle’s birthstone is a diamond; Debra’s is an opal.

B: My birthstone is a ruby; not a diamond.

C: Diamonds are popular; My birthstone is a ruby.

D: Michelle’s birthstone is a diamond; but Debra’s is an opal.

I thought it was B, but I'm not sure. Can someone who knows what the right answer is please tell me if I'm right (or correct me if I'm wrong and explain the correct answer)?