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Opal

Streak: White

Hardness: 5 – 6

Chemical Formula: SiO₂·nH₂O

Opal is formed from a solution of silicon dioxide and water. As water runs down through the earth, it picks up silica from sandstone, and carries this silica-rich solution into cracks and voids , caused by natural faults or decomposing fossils.

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

Garnet (Almandine)

Streak: white

Hardness: 7 – 7.5

Chemical Formula: Fe3Al2(SiO4)3

Garnets are a group of silicate minerals. Almandine is the most common of the garnet group and always features a red color.

Crystals of almandine often have well-developed faces and complex crystalline structures. Due to its color, almandine garnets are frequently cut and used as gemstones, but coarse varieties are often crushed for use as an abrasive.

Photo: Almandine by Rock Currier is licensed under the Creative Commons Attribution 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.

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Celeste
Author: Celeste

Rocks!