Suppose, for example, that a mineral can be scratched by [fluorite], which has a hardness of 4 on Mohs scale, but cannot be scratched by [calcite], which has a hardness of 3. We then know that this mineral is softer than fluorite, but harder than calcite; therefore, it has a hardness of about 3½. In the same way, if a mineral can be scratched by a pocket knife, which is slightly more than 5 in hardness, but not by a copper penny, which has a hardness of about 3, we know then that its hardness is between 3 and 5.
[Streak] or Powder
The [streak] is the mark, made of fine powder, that a mineral leaves as you rub it across a streak plate. A streak plate is a flat piece of white tile or porcelain that has a dull, unglazed surface. The streak plate is about as hard as [quartz], which is 7 on Mohs scale, and you will not be able to use it for minerals that have a greater hardness. For these, you can obtain the powder by scratching the mineral or by crushing a small piece of it.
A [streak] plate is used to determine the color of the streak or powder of a mineral.
The color of the [streak] or powder is extremely helpful in identifying some minerals. For example, [hematite] is a mineral that may be any one of several different colors, but its streak or powder is always reddish brown.
[Cleavage]
As they break, some [crystalline] minerals always split along a smooth, flat surface. This property is known as [cleavage]. Some cleavages are smooth and perfect; others are not so perfect. The cleavage surfaces, because of the mineral’s crystalline internal structure, are parallel to possible crystal faces, even though the mineral itself may occur as a crystalline mass without a perfect crystal shape.
Some minerals will cleave in only one direction; some, in several directions. For example, [galena], a mineral found in Texas, has perfect cubic [cleavage]. It cleaves in three directions that are at right angles to each other. These cleavage directions are parallel to possible cubic crystal faces, and some of the [cleavage fragments] are [cubes].