Let's Go Rock Collecting, by Roma Gana, illustrated by Holly Keller, HarperCollins, 1997.
My son used this book for his Second Grade Science class. The text contains some solid information, and it is accessible to a second-grader; however, this book did not prove to be nearly as helpful as I hoped. I think the problem is with the illustrations. The illustrations, even of the rocks, are a bit cartoonish, which made it difficult for my son to match up his rock sample to the correct rock.