All 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.
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
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
|
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.