Which of the following statements about gemstones is true? Choose one:

A. Emerald is a gemstone variety of the common mineral corundum.
B. In order to be considered precious or semiprecious, a gemstone must be transparent.
C. A gemstone is a variety of a mineral that has value because of its color.
D. Many gemstones come from pegmatites, which are particularly coarse-grained igneous rocks.