Gems for Kids, A Junior Scientists Guide to Mineral, Crystals, and Other Natural Treasures by Lee Hall and Ashley Hall is a great book for young scientists interested in gemology or for those who want to learn more about the beautiful minerals.
It has information about up to 25 different types of gems, from diamonds to moonstone to pearls!
Gems for Kids has very clear sections that differentiate gems along with facts that make them easily identifiable making it easy to absorb, and even including a project for junior scientists to grow their own crystals!
$16 (includes shipping) plus tax!