Color is one of the physical properties most commonly used to describe minerals. Color is not always a good clue to identity of a mineral. Many minerals are found in several colors and many minerals have extra chemicals in them that give them an unexpected color. Also weathering may change the color of the mineral. Most minerals have a distinctive color that can be used for identification. But color can narrow down the number of minerals that it could be to a smaller amount.
Luster is a description of the way a mineral surface looks when light reflects off of the surface. Luster has two categories, metallic and non metallic. Metallic luster refers to minerals that look like a shiny metal. Some examples are pyrite and magnetite. Nonmetallic luster is not looking like a metal at all. Some examples are greasy, silky, and pearly. The luster of a mineral is affected by the brilliance of the light used to observe the mineral surface. Luster simply means the way that lights reflects off a mineral light can make a mineral look very dull or as shiny as a diamond.
Streak is the color of the mineral when it is crushed to a powder. When testing for streak, the mineral must be crushed to determine the color of its powder. The color of the powder is the color of the streak. Instead of actually crushing a mineral to determine the streak, it is much simpler to swipe the mineral across a streak plate. A streak plate is an unglazed piece of porcelain, such as the underside of a ceramic tile. This is the most popular method of streak testing, since the color of the streak plate is white, the color of the mineral trace is easy to see.
Cleavage refers to the way some minerals break along certain lines of weakness in their structure. If a mineral breaks along flat, smooth surfaces it shows cleavage. Cleavage can be in one, two or three directions. Some examples are:
Mica- cleavage in one direction,
Feldspar- cleavage in two directions,
Calcite, Galena and Halite- cleavage in three directions,
If a mineral breaks along irregular rough surfaces it shows fracture. Quartz shows a special type of fracture called concoidal (shell-like) fracture.
Some minerals form crystals, if there is time and room for the crystals to form. The crystal pattern of a mineral is controlled by the internal arrangement of the atoms that make up the mineral. Some examples of these crystal structures…