Crystals and Cleavage.   When you see a smooth, flat surface on a mineral it can be either a crystal face or a cleavage plane.

Crystals: First, a little chemistry.   All matter in the universe is made up one or more types of elements or atoms.   There are about 90 or so elements that occur in the universe naturally.  Some are more common than others.  In the Earth's crust, the most common elements (atoms) are oxygen, silicon, aluminum, iron, calcium, sodium, potassium and magnesium.  Only two element, oxygen and silicon, make up about 75% of the earths crust! 

An atom of oxygen is different than an atom of silicon, which is different from an atom of iron.  Each type of atom is a different size, weight, and has very specific electrical properties. There is a special chart that shows all the known elements, including their weight and electrical properties, called the Periodic Table.

Atoms are often attracted to other atoms that have certain electrical properties.  For example, one oxygen atom often combines with two hydrogen atoms. The result is water.  You may have heard people call water "H₂0".   The "H₂" stands for "2 hydrogen atoms" and the "O" stands for "one oxygen atom."  When two or more atoms are combined together they are called molecules.

The atoms in these molecules are held together by bonds. Bonds are sort of like electrical glue. Some bonds are strong, and some are weak.   Covalent bonds are very strong.  It's hard to break them.  Ionic bonds are weaker.  Minerals with ionic bonds tend to dissolve in water and are called "salts."  A third type of bond is the Vanderwall bond, and it is very weak.    Some molecules have more than one type of bond.

Crystals have a repeating pattern of molecules or atoms, and this is what makes crystals different from other types of matter.   

Graphite is the natural form of carbon at normal pressure and temperature.  Thin sheets of carbon atoms form.  Within the sheets are strong covalent bonds.  However, the sheets are attached to other sheets by very weak Vanderwall bonds.  The sheets split apart from each other very, very easily. Therefore, the mineral is very soft and is used as pencil lead and as a lubricant.

Diamond, on the other hand, is an unusual form of carbon that forms only under extreme pressure and heat. Instead of forming sheets, each carbon atom forms four strong covalent bonds with four other carbon atoms. The result is the hardest, most durable mineral on the planet.

Minerals have distinctive crystal appearances, yet two crystals of the same mineral often look very different from each other. If you examine the crystals carefully, though, you would find that the angles of the crystal faces are the same.  Mineral crystals can be identified by their symmetry, but this is not for beginners.  Instead, look at the general appearance of the crystals, or "habit".  For example, crystals might be fibrous, needle-like, prismatic, square, or grow in sheets like mica.

Cleavage: When you break a mineral in half, it might break along a smooth, flat plane.   This is a cleavage plane: A weakness in one directly that occurs throughout the mineral because of its internal crystal structure.  The mineral break where the bonds are weakest.  If the plane is very smooth and glassy, the mineral has "perfect cleavage."   Some minerals have cleavage in one, two and even three directions.  Calcite is a great example.  Halite, or rock salt, is another mineral that breaks easily in three directions.  Feldspar also tends to break in two or three directions, although not quite so smoothly.

Cleavage planes are always parallel to a crystal face or a potential crystal face in a crystal. 

Other minerals don't have any cleavage at all.  Quartz will never break along a flat plane.  For these minerals we describe the "fracture" surface.  For quartz, it is concoidal, which means it looks similar to broken glass.