Linda Kirk, age 12, of Peterborough, Ont., Canada, for her question:
What is the hardest mineral in nature?
The natural minerals we know are the rocky and metallic substances of the Earth's crust. These upper layers of the solid Earth reach down only 10 to 40 miles, and no one has been down to Explore what materials lie beneath them. Down in the depths of our solid planet there may or may not be minerals much harder than those we know on the surface.
Every rock hound knows that hardness is one of the tests used to identify a new specimen from the crusty ground. The classes of mineral hardness run from 1 to 10, with degrees of difference in each class. Sulphur has a hardness of exactly 2. Borax, with a hardness of 1.7, is between classes 1 and 2, while glassy topaz, with a hardness rating of 3.5, is midway between classes 3 and 4.
The way to test the hardness of a new rock sample is to scratch it. A lump of calcite in class 3 can be used to scratch gypsum of class 2 and talc of class 1. A mineral from a harder class can scratch all the minerals in the softer classes, but the softer minerals cannot scratch a harder mineral back. There are dozens of minerals in classes 1 to 9, each one harder than the one having the next smallest number.
It would take a lifetime to test them all. But finally we would find one mineral able to scratch every other natural mineral of the Earth's crust. And no other natural mineral can scratch it back. This mineral is the diamond that occupies class 10 on the mineral scale of hardness all by itself.
A diamond is a crystal form of the soft element carbon. It is made from the same atoms as coal and graphite, which are sooty black and not hard as minerals go. Experiments suggest that the Earth's diamonds were formed from carbon materials, heated to perhaps higher than 5,000 degrees Fahrenheit, where pressure was at least a million pounds upon every square inch. Such conditions could exist at depths of some 250 miles below the crusty surface of the globe. Nature’s diamonds, we suspect, may be shifted from their deep levels by the molten magma that forms around the roots of volcanoes. Many of them have been carried upward with erupting lava and left to cool in the material that later plugged the volcano's vents and pipes. Every year, more than four tons of the Earth's hardest mineral are mined from the rich diamond deposits of Africa. About 80% of these diamonds are used in industry to cut and polish natural and man made minerals softer than themselves.
In spite of its hardness, diamonds are one of nature's lightweight minerals. A great many substances are much heavier. A thimbleful of iron has somewhat more than twice the weight of a thimbleful of diamond. The silvery metal platinum outweighs the diamond by more than six to one. A cupful of osmium, the densest of all nature's minerals, is almost seven times heavier than an equal amount of hard hard diamond