What Is the Hardest Stone?

General Information

• According to the United States Geological Survey, diamonds are the strongest and hardest known material in the world, scoring a maximum of 10 on the Mohs Hardness Scale. They are made of pure carbon. Because of their hardness, diamonds are also the longest-lasting of all gemstones, according to Mark Helper from the University of Texas. Diamonds do not dissolve in acid, but they can be destroyed if subjected to intense heat. When a diamond is heated above 1,100 degrees Fahrenheit in the presence of oxygen, it will burn and form carbon dioxide. If it is heated to about 1,300 degrees without oxygen, it becomes graphite.

Uses

• Diamonds' hardness allows them to be used for several purposes besides making jewelry. Diamonds that do not meet gem-quality standards for color, clarity, size or shape are called "industrial diamonds." These are used to cut, grind and bore other hard materials, such as metals used to make cars, airplanes and other machinery. These diamonds are also placed on the end of mining drills. Diamonds can only be cut by other diamonds, but they can be cleanly broken with a sharp, accurate blow because of their cleavage. Cleavage is the property that allows some stones to split in certain directions and produce flat surfaces. Diamond cutters have been using iron wheels coated with diamond dust to shape and polish stones to make jewelry since the 1400s, according to Helper.

Why Are Diamonds So Hard?

• Diamonds are extremely hard because of the tremendous chemical bond between each carbon atom. Moreover, the atoms in diamonds form a rigid structure, such that each atom is connected to four others, creating a regular and sturdy network that is almost indestructible.

Other Hard Stones

• According to the American Federation of Mineralogical Societies, corundum is the world's second hardest stone, scoring a 9 on the Mohs Hardness Scale. It is 25 percent as strong as a diamond. The transparent varieties of corundum include rubies and sapphires. Nontransparent corundum is used to grind, smooth and polish other hard materials. Topaz, scoring an 8 on the Mohs Hardness Scale, is the world's third hardest stone. Emerald and aquamarine are common varieties of topaz, which is half as hard as corundum.