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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.

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.

sample of smoky quartz

Quartz

Hardness: 7

Streak: White

Chemical Formula: SiO2

Quartz is one of the most common minerals found in the Earth’s crust. While it is usually colorless and transparent, quartz can also be found in a wide variety of colors and a range of opacity. These varieties generally have their own name such as rose quartz, amethyst, smoky quartz, and citrine.

Quartz can also be found in cryptocrystalline varieties, or quartz that is made up of microscopic crystals. These varieties include chalcedony, agate, and jasper.

Gold

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.

sample of pyrite

Pyrite

Streak: Greenish-Black

Hardness: 6 – 6.5

Chemical Formula: FeS₂

Pyrite is the most abundant sulfide mineral. It’s pale brass-yellow color is where pyrite gets its nickname “Fool’s Gold” as it was commonly mistaken for gold by novice prospectors.

The name pyrite is derived from the Greek phrase pyrites lithos, “stone or mineral which strikes fire,” due to the fact that pyrite emits sparks when struck by iron.

Pyrite is well known for it’s cuboid crystal habit, but it can also be found in many other forms. Pyrite can form dodecahedral, or twelve sided, forms known as pyritohedra.

Fool’s gold can be differentiated from real gold by its hardness, brittleness, and crystal form. Pyrite’s streak also is a defining characteristic as it is generally greenish black to brownish black whereas gold tested on a streak plate will leave a yellow streak.

Pearl

Streak: White

Hardness: 3

Chemical Formula: CaCO3

Pearls are an organic mineral formed inside a mollusk. The mollusk takes aragonite, the same mineral that makes up its shell, and builds up concentric layers around any foreign particle that enters the mollusk’s mantle.

Pearls vary widely in color and shape, depending on its environment and also the shape of the foreign object that entered the mollusk. Farmed pearls achieve their perfect, spherical shape by planting perfectly spherical “seeds” into the shells and allowing the mollusk to build up only a small amount of aragonite secretions to cover the item.

Photo: Freshwater Pearl by Jennnifergaglione This file is made available under the Creative Commons CC0 1.0 Universal Public Domain Dedication

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Celeste
Author: Celeste

Rocks!