The allure of a diamond makes it the most highly revered among gemstones. It has many qualities that make it so desirable and beloved, not the least of which is its rarity.

Diamonds are the hardest, the most brilliant and the most imperishable of all precious stones. The word “diamond” comes from the Greek word “adamas,” meaning unconquerable.

Their clarity, brilliance and fire are rivaled by no other gem, yet they are made from carbon, one of the most remarkable chemicals in the periodic table. It is the blackness in a lump of charcoal and is also the chemical element responsible for life itself.

Diamond is carbon in its most concentrated form and, except for trace impurities like boron and nitrogen, which give it tints of pink and yellow, its purest form as well.

Diamonds have been a source of fascination for centuries. From myths about valleys of diamonds protected by snakes, to the production of millions of carats in rough diamonds each year, the history of diamonds dating back some 3,000 years to India is one of mystical power, beauty and commercial resourcefulness.

Only a few diamond deposits were known until the 20th century, when scientific understanding and technology extended diamond exploration and mining around the globe.

Today diamonds are mined in about 25 countries, on every continent but Europe and Antarctica. Surprisingly, more than one-third of the world’s diamonds come from Australia.

Diamonds are formed deep within the earth, in the mantle between 60 and 120 miles below the surface where temperatures are 1,700 to 2,400 degrees Fahrenheit and the pressure is 45,000 to 60,000 atmospheres.

The youngest known diamonds were formed 1 billion years old, and many are more than 3 billion years old.

VOLCANIC eruptions carry diamonds to the surface via a magma conduit known as a kimberlite pipe. Most diamonds occur at the surface in an ordinary-looking volcanic rock known as kimberlite.

The kimberlite magmas that carry diamonds to the surface are much younger than the diamonds they carry. The preexisting diamonds simply ride the kimberlite magma like an elevator.

Diamond is the hardest known mineral, and graphite is one of the softest. The reason for this is that carbon atoms in diamond are linked together in a 3D network, while in graphite the atoms form sheets that are held together loosely, allowing the sheets to slide past each other easily.

Diamonds are rare even in kimberlite, occurring at an average rate of 1 gram of diamond per 200,000 grams of kimberlite.

Only 20% or fewer are of gem quality. The rest fall into two other categories known as near-gem quality and industrial-quality diamonds.

Near-gem quality diamonds represent those stones of a quality between gem and industrial, that in fact can be used as either depending on the individual stone.

THE DISTINCTIVE fire and brilliance that give diamonds their extraordinary appeal are due to the strong chemical bonds between carbon atoms.

Carbon atoms have four bonding sites and have an uncanny facility to form bonds with other carbon atoms by sharing electrons at those bonding sites.

This forms the most perfect of covalent bonds, wherein the electrons are shared equally. In most covalently bonded molecules, such as water, the probability of finding an electron is greater near the oxygen atom than either of the two hydrogen atoms. In a covalent carbon-carbon bond, electrons are equally distributed among the bonded atoms.

In the perfect covalent bonds of the diamond lattice, the carbon atoms form a 3D interlocking lattice of tetrahedral diamond structures.

These bonds are likewise responsible for the primary optical properties that give diamonds their distinctive fire and brilliance.

Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.