Upcoming Rockhounding Events in California

Nov 06

Contra Costa Mineral & Gem Society

November 6 - November 7


Streak: golden-yellow

Hardness: 2.5 – 3

Chemical Formula: Au

Gold generally occurs in tree-like growths or grains. Because of it’s inert state, gold resists tarnish and is generally found in a relatively pure form.

Almost all of the gold recovered from the earth comes from placer deposits, weathered particles concentrated in river gravel. This relatively rare mineral has been used since antiquity in many facets of human life from coinage to art and beyond.

Today, 10% of the gold produced finds its way into industry with much of it being used to produce electronic components.

Photo: Gold by Robert M. Lavinsky is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.


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.


Streak: colorless

Hardness: 7 – 7.5

Tourmaline is the name given to a family of minerals sharing the same basic crystal structure. Some of the minerals in this group are elbaite, schorl, dravite, and liddicoacite. Gemstone varieties based on their color are also recognized such as rubellite, pink or red, and verdelite, green.

Most tourmaline is dark and opaque but many of its varieties are valued as gems.

Photo: Tourmaline by Robert M Lavinsky is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.

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Author: Celeste