Montana designated both the sapphire and Montana Agate as the official state gemstones in 1969. Agates are found in southern and eastern Montana. Agate is polished (not cut) to make gemstones and jewelry. Agates are usually white with swirls of grey and black spots.

Montana's sapphires (mostly found in western Montana) look like bright, blue glass and are cut like diamonds to make jewelry. Montana sapphires can be found in the Royal Crown Jewels of England. Sapphires, which were thrown away by miners in the gold rush days, are now the most valuable gemstones found in America.

Source:

Montana State Symbols: MontanaKids

Links:

Geological Symbols - 50 States List