What You Need to Know About Minerals & Gemstones

Minerals are unique chemical substances which are homogeneous (the same) throughout the specimen.  Minerals can be found as single crystals or clusters of many crystals.  Rocks are a group of minerals that are found together.  The type of rock is determined by the type of minerals that are formed together along with the place where the formation occurs, such as deep in the Earth’s crust or near the surface.  For example, quartz, feldspar and mica are individual minerals, but when they are found together in a rock that formed underground, it is often called granite.   Some minerals are very common and some are rare.  However, even the most common minerals can have unique and rare forms.  Some minerals are useful in the production of industrial materials such as gypsum in cement, mica in paints and coatings, feldspar in ceramics, and quartz in watches and other electronics.  Other minerals are used for jewelry and are considered precious (diamond, emeralds, and ruby) or semi-precious (amethyst, citrine, garnet, and peridot) gemstones based on how easy they are to find.  

Mineral Color:  The Natural Color of a Mineral
Minerals come in many colors.  Some minerals can form in one single color, like royal blue sodalite, or can form in many colors such as fluorite which can be blue, red, purple, yellow green or white.  When geologists speak of the mineral’s color, they are talking about the outward appearance of the mineral itself.  

Mineral Streak:  The Color in Powder Form
The streak color is the color of the mineral in powder form. Streak is a useful tool in determining a mineral’s identify because some minerals will streak the same color as their outward appearance and some will streak a completely different color.  Mineral streak is tested using “streak plate.”  The mineral is rubbed on the plate during the test.  The color that shows on the plate is the streak color.   

Mineral Luster Luster is the appearance of a mineral when the light shines on the sample.  Minerals can have different luster which is why it is another clue to the identity of a mineral.  There are many different mineral lusters.  Some of the most common lusters are glassy, greasy, silky, earthy, metallic, and pearly.

Mineral Hardness:  The Mohs Hardness Scale

The hardness of a mineral is often used by geologists to help determine the identity of a sample. The Mohs Hardness Scale as developed by a German geologist, Friedrich Mohs, in 1812. The Mohs scale is a relative scale which lists the hardness of 10 common minerals. Talc, #1 on the scale is the softest and diamond, #10, is the hardest. Other common household items have also been assigned to the hardness scale such as glass, fingernail and a penny. The Mohs mineral scale is:

Hardness Mineral
1 Talc
2 Gypsum
3 Calcite
4 Fluorite
5 Apatite
6 Orthoclase Feldspar
7 Quartz
8 Topaz
9 Corundum
10 Diamond

The first nine minerals on the Mohs’ Hardness Scale have nearly the same relative hardness between them. For example, fluorite is four times harder than talc, quartz is seven times harder than talc and corundum is nine times harder than talc. However, the tenth mineral on the scale, diamond, is 40 times harder than talc.  

Mineral Shape
Minerals can either form in masses with no distinct shape or in various crystal geometries.  Some of the most common shapes are cube, octahedron, rhombohedron, prismatic and platy.

There is more information available about rocks in minerals in the Help, I Have to Teach Rock and Mineral Identification and I’m Not A Geologist! book.

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