Properties of Minerals

Science background provides teachers with more detailed information about the phenomena students explore in this unit. Here is an excerpt from the science background information on Earth materials. 

All diamonds have the properties they do because of their atomic makeup and structure. Diamonds are minerals, which means they are naturally occurring, inorganic solids with a crystal structure.

Carbon atoms in diamonds form extremely strong bonds in the shape of a three-sided pyramid, called a tetrahedron. This creates a rigid lattice that doesn’t allow the atoms to move. This structure causes diamonds to be extremely hard. Hardness is a measure of a mineral’s resistance to scratches. Mineralogists use a 1-10 hardness scale called the Mohs scale. Talc is a 1, which means it is very soft. Diamond, in contrast, is a 10, making it the hardest naturally occurring mineral. It is so hard that only another diamond can scratch it.

The hardness of diamonds makes them useful for many different applications because they can be used to cut other materials including stone, metal, and concrete. For example, they are used in cutting tools for military materials such as airplane parts and armor.

Diamonds’ atomic structure also causes them to have a high melting point. Diamonds are also excellent thermal conductors, which means they conduct heat.

Scientists rarely rely on just one property test when they are identifying different minerals. Many minerals share the same physical properties, including color and hardness. However, when many of a mineral's properties are examined, it often results in a unique set of properties that can be used to correctly identify the mineral.

For example, in addition to hardness and conductivity, diamonds also often have a high luster. Luster is the way light reflects off of the surface of a mineral, and how bright, dull, or greasy the mineral appears. Luster falls into two categories: metallic and nonmetallic. Cut diamonds often have a metallic luster. This gives them a brilliant, freshly polished look. In contrast, uncut diamonds can have a greasy or dull luster.

Pure diamonds are also the most transparent known material. Transparent substances allow light to pass through. Pure diamonds are rare, however. Some non-pure diamonds are translucent, which means they allow some light through and absorb some light. Opaque diamonds absorb all light.

All diamonds are formed deep within Earth, at least 161 kilometers (100 miles) below the surface. They can only form under tremendous heat and pressure deep within Earth’s structure. At temperatures reaching 1,200 degrees Celsius (2,192 degrees Fahrenheit) and under deep pressure, carbon atoms come together within Earth’s interior to link in a crystal structure to form diamonds. Earth’s interior is powered by heat that originates in Earth’s center, called the inner core. The inner core is a solid ball of metal iron and nickel that reaches 7,000 degrees Celsius, hotter than the surface of the sun.

The inner core is surrounded by the outer core, which wraps around the inner core and is made of liquid iron and nickel. This metal gives Earth a magnetic field, which shields the planet from the sun’s radiation.

The mantle is located around the outer core and is mostly molten, semi-solid rock called magma. The mantle makes up almost two thirds of the Earth's mass and is about 2,900 km (1,802 miles) thick. It is here where diamonds are formed. Earth’s crust is the hard rock layer of the planet that supports the continents and oceans.