From Talc to Diamond: Moh's Scale of Hardness
The resistance of minerals to being scratched or abraded is known as the mineral's hardness. This hardness has been compared and tested as far back as 300 B.C. by the Greek philosopher Theophrastus and in 33 A.D. by the Roman author and philosopher Pliny the Elder. Both knew that the hardness of minerals could be determined by scratching one mineral against another. In 1812, a German mineralogist went a step further and created a hardness scale to use when testing mineral hardness. This scale is known as the Mohs Hardness Test, and it was designed by, and named after, Friedrich Mohs. Like Pliny the Elder and Theophrastus, Mohs knew that when rubbed together, the hardest of two minerals would leave a scratch on the other. He numbered his scale from one to 10 going from the softest to the hardest mineral. Each number on the scale is represented by a mineral and its hardness value. The minerals listed are meant to be used as a standard means to test and determine the hardness of other minerals. Although the scale was designed to measure the hardness of minerals, it is not meant to measure the exact degree of hardness. That is because the actual hardness of a mineral is based on factors such as the amount of crystallinity and its purity. In addition, it is also not meant to be a test of brittleness, which differs from hardness. In some cases, a mineral may be harder than others, like a diamond for example, but it may be extremely brittle as well. The following is the Mohs scale in order of softest to hardest mineral:
- 1. Talc: Talc is rated as the softest of the minerals in general. Although talc is popularly recognized as talcum powder, it may also be used for a variety of other products, such as cosmetics.
- 2. Gypsum: This is also a soft mineral. It is composed of sulfate dihydrate. Its uses include wallboard, Plaster of Paris, cement, and fertilizer.
- 3. Calcite: Calcite is a mineral that is commonly used in animal feeding as a dietary supplement and can be found in antacids for humans. It is in marble and limestone rocks and can be found hanging from the walls of caves. It is a carbonate and a highly distributed mineral that may appear as different forms of crystals.
- 4. Fluorite: Fluorite is a mineral that can be found in various places worldwide. It has many industrial uses and is used frequently in common items such as enamel, lenses for cameras and telescopes, and in steel manufacturing. In appearance it can be found in various colors.
- 5. Apatite: Apatite is commonly used in fertilizer because it is a source of phosphorous. The term apatite refers to a group of phosphate minerals. These are fluoraptite, hydroxylapatite and chlorapatite. It can be found in colors such as violet, green, yellow, or brown. It may also appear colorless.
- 6. Orthoclase: Orthoclase is frequently used as an ingredient in porcelain and porcelain items, such as dolls. It is also widely used in glass and ceramics. Orthoclase also goes by the name of Feldspar.
- 7. Quartz: This is a highly recognizable mineral that is used for numerous purposes, including as a gemstone for jewelry. Historically it was used to create arrowheads due to its hardness. It is highly heat-resistant and is used in bricks for this reason. Other uses for quartz include the making of fiberglass and glass. Different types of quartz include citrine, amethyst, rose quartz, and smoky quartz.
- 8. Topaz: This mineral is a very common gemstone that is frequently used in jewelry. It can be found in a host of different colors such as brown, blue, yellow, pink, and orange. Much of its popularity stems from its clarity and hardness.
- 9. Corundum: Corundum is the second hardest mineral behind the diamond. Both rubies and sapphires are types of popular corundum used in jewelry. Both rubies and sapphires are precious stones. Corundum also has its industrial uses and is used in sandpaper and cutting tools.
- 10. Diamond: The hardest mineral is also one of the most beautiful and sought after in the world. While most people will associate diamonds with jewelry such as necklaces, rings, or earrings, their hardness makes them a valuable tool as well. Diamond particles can be used in tools made for cutting, grinding or drilling. When in powder form, it has also been added to polishes.
The minerals that are used may not always be readily available for testing purposes. For this reason, there are alternative objects that can be used in the place of the ten items in the Mohs Hardness Scale. These items also have a specific hardness level or number that is associated with them for testing. They are items that people typically have in their possession, which makes them ideal for use as a demonstration or as a project for children. Objects that are commonly used for testing include fingernails, a copper penny, a knife made of steel or a glass plate, and a steel file. The hardness of these items ranges from 2.5 for a fingernail and up to 6.5 for a file that is made of steel. Between the least and hardest objects on the alternate list is a penny, which has a standard hardness of 3. A glass plate has a hardness value of 5.5. When using these objects, the hardness of the mineral that is being tested can be determined up to the scales nearest half number.
Performing the hardness test is relatively simple, but there are certain rules that should be remembered. The first is that if two objects are of the same hardness, they may be scratched but with difficulty. The second principle to remember is that the mineral or object that creates the scratch is always harder than the object that receives the scratch. Keeping these in mind, the person performing the test should start with a mineral or object that is at the softest end of the scale. This may be one's fingernail or talc depending on which test objects are being used. Beginning with the softest of the objects allows for the simplest process of elimination. If the object does not scratch, then the next hardest object is used until a scratch is made. When attempting to scratch the surface, the tester will ideally want to save the surface and quality of the mineral in question. To do this, the mineral being tested should be used to scratch the surface of the penny, glass plate, or other test objects such as the minerals listed on the Mohs scale.
- American Federation of Mineralogical Societies: Mohs Scale of Mineral Hardness
- Moh's Hardness Scale
- The "Microhardness" of Minerals Comprising the Mohs Scale
- Testing the Hardness of a Mineral
- Sedona Gem and Mineral Club: An Updated and Extended Mohs Mineral Hardness Scale
- Hardness of Minerals I: The Mohs Scale (PDF)
- The Hardness Scale Introduced by Friederich Mohs
- Hardness of Minerals
- The International Lapidary Association: The Hardness of Rocks and Minerals
- Mineral Identification: Hardness
- Central Michigan Lapidary and Mineral Society
- Rocks and Minerals Make Up Your World PDF
- Minerals Matter: How to Identify Minerals
- Material Hardness
- Introduction to Minerals and Rocks