Rockhounding in Montana – What Rocks You Can Find and Where

Sapphire (Corundum)

Streak: Colorless

Hardness: 9

Chemical Formula: Al₂O₃

Sapphire is a precious gemstone, a variety of the mineral corundum, consisting of aluminum oxide with trace amounts of elements such as iron, titanium, chromium, vanadium, or magnesium. It is typically blue, but natural sapphires also occur in yellow, purple, orange, and green colors.

It is common practice to heat natural sapphires to improve or enhance their appearance. Evidence of sapphire and other gemstones being subjected to heating goes back at least to Roman times.

Photo: Several Corundum Crystals by Ra’ike is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.

Agate

Streak: White

Hardness: 7

Chemical Formula: SiO₂

Agate is a common form of chalcedony, a microcrystalline form of quartz. It’s banded, colorful appearance has lead to its widespread use throughout human history.

Most varieties of agate are named for its visual appearance, such as fire agate which has inclusions of red or brown hematite. Agate can also vary in its banding appearance, some agates have a dendritic habit or lace-like patterns with eyes, swirls, and other patterns.

Because this mineral is a form of quartz, it’s hardness lends itself to uses not commonly associated with such visually appealing minerals. Agate has been used to create precision pendulums, mortars and pestles, and fine knife-bearing edges for laboratory balances.

Photo: Agate by James St. John is licensed under the Creative Commons Attribution 2.0 Generic license.

Smoky Quartz

Streak: White

Hardness: 7

Chemical Formula: SiO₂

Smoky quartz is a brownish grey, translucent variety of quartz that ranges in clarity from almost complete transparency to an almost-opaque brownish-gray or black crystal. Like other quartz gems, it is a silicon dioxide crystal. The smoky color results from free silicon formed from the silicon dioxide by natural irradiation.