Idaho's nickname is The Gem State because of its abundance of natural resources and scenic areas that include snow-capped mountain ranges, rapids, vast lakes and steep canyons. The waters of Snake River rush through Hells Canyon, the deepest canyon in the United States. Shoshone Falls plunges down rugged cliffs from a height greater than that of Niagara Falls.

The mountains of Idaho contain veins of gold, silver, lead, zinc, cobalt, copper, and many other rare minerals. Among these rare minerals are gems - star garnets (the state gem), jasper, opal, jade, topaz, zircon, and tourmaline.

Source:

Idaho State Symbols: Idaho.gov
Idaho - A Portrait - Geology of Idaho: Idaho Public Television
Idaho: Wikipedia

Links:

State Nicknames - 50 States
Geological Symbols - 50 States