Diamonds are still a girl's best friend, right? We love the shiny gems. They are the most popular rocks sold today. But what exactly are they, anyway? Where do they come from? What else are they used for?

Diamonds are a mineral in one of the two crystalline forms of the element carbon. They are the hardest natural substance man knows. Diamonds are sold as gems, and used in industrial applications for smoothing, cutting, and polishing hard materials.

Diamonds are most famous for crystallizing in the common colorless form. They may also be tranlucent to transparent white, yellow, green, blue, or brown. Diamonds have a high refractive index which is why they are so brilliant and sparkly after cutting. The familiar shape of the diamond is the octahedron.

The most brilliant diamonds become gemstones for jewelry and other uses. For those that don't make it to gems, there are other options. There is bort, which is a more poorly crystallized or undesirable color and in fragmentary condition, and carbonados which is gray to black opaque. Bort and carbonados are used as abrasives for the cutting of diamonds and the cutting heads of industrial rock drills.

Diamonds are found in alluvial formations and in volcanic pipes, filled for most of their length with blue ground or kimberlite, and igneous rock consisting primarily of serpentine. Diamond yielding earth is mined by both the open-pit method and by underground mining. After removal to the surface, the soil is crushed and concentrated. Passing the concentrated material in a stream of water over greased tables does the needed sorting. The diamond is largely water repellent and sticks to the grease and the other minerals retain a film of water, which prevents the sticking to the grease. Then the diamonds are removed from the grease, cleaned, and graded for sale and use.

The earliest sources of gem diamonds were India and Borneo. Some famous diamonds are the Great Mogul, Regent, and Pitt. Other famous diamonds include the Hope (blue), Dresden (green) and Tiffany (yellow). In the early 18th century, deposits similar to those in India were found in Brazil, mainly of carbonados. In 1867 a stone found in South Africa was recognized as a diamond. Within a few years began a wild search for diamonds. In 1870-1871, dry diggings including most of the celebrated mines were discovered.

Synthetic diamonds were successfully produced in 1955; a number of small crystals were produced when pure graphite mixed with a catalyst was subjected to pressure of about 1 million lb per sq in. and temperature of the order of 5,000-F (3,000-C). Synthetic diamonds now are extensively used for industry, mainly due to the ease of obtaining and lower cost for them. Diamonds are still very popular and symbolize many things. Their popularity does not seem to be dwindling any time in the near future.