Some minerals are soft and can be scratched easily. Others, which are harder, are resistant to scratching. To measure a mineral’s hardness, we try to find out which substances will scratch it and which substances will not scratch it. To do this in a general way, several ordinary objects—such as a fingernail, a copper penny, a pocket knife, a piece of window glass, and a steel file—can be used. For a more exact way of testing hardness, we can use ten minerals that make up what is known as _Mohs scale_. Each mineral in this scale has a different hardness, and each one has been given a number that represents its hardness. For example, talc, the softest mineral in this scale, is given a hardness of _1_. Gypsum, the next softest mineral in the scale, has a hardness of _2_. Diamond, the hardest mineral known, is given the top hardness of _10_ in this scale. These ten minerals are listed below. Alongside them are five common objects with their hardnesses. 1—Talc 2—Gypsum Fingernail—slightly over 2 3—Calcite Copper penny—about 3 4—Fluorite 5—Apatite Pocket knife—slightly over 5 6—Orthoclase Window glass—5½ 7—Quartz Steel file—about 6½ 8—Topaz 9—Corundum 10—Diamond 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. [Illustration: 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