Under very specific instructions, a jeweler made one ring for the queen and one ring for the princess. The gem on the queen's ring weighed 1/2 of a carat and the stone on the princess's ring weighed 1/8 of a carat. How much heavier was the stone on the queen's ring than the stone on the princess's?

a. 3/8
b. 4/8
c. 5/8
d. 1/2