Diamond is probably the best known gemstone consisting entirely of carbon, whose hardness and high dispersion of light make it useful in many other applications such as industrial. Diamond is probably best known as one of the most beautiful gemstones and has the largest amount of scintillation (fire) of any gem and appears like something similar to the twinkling stars in the night sky. Diamond also has 4 times the hardness of the next hardest mineral, corundum. Even though diamond is by far the hardest known material that cannot be scratched by any other except its own, it does have the ability to fracture based on its cleavage due to its particular crystal structure, which has certain planes of weakness. Diamond is said to have perfect cleavage and can split in four different directions instead of breaking in a jagged or irregular fashion. Diamonds are unique in that they repel water, but adhere to wax and grease, which provides an efficient means to separate diamonds from other minerals that come out of mining operations, which are flushed over a sloping surface covered with a mixture of wax and grease. This allows the diamonds to stick to the grease and wax. Diamond is the oldest material known, weighing in at around 3 billion years and is formed in the Earth's mantle, being shot up into volcanic vents and volcanoes. Diamond also has a high degree of color ranges and exhibits colors like blue, pink, and yellow. Blue diamonds get their color from boron while yellow diamonds are colored by trace amounts of nitrogen. Red diamonds are the rarest gemstones in existence with only two in the known world.

Diamonds have been known since antiquity and were called by the ancient Greeks, "adamas" (invincible"). The ancients even used diamonds for industrial applications such as drill bits and engraving. The history of diamond mining, like many other gems, goes back to ancient India in 4000 B.C. and continued to be the dominant supplier until in 1725 when diamond veins were discovered in Brazil and then over a hundred years later in South Africa. Now other major producers include Australia, Botswana, Russia, Zaire. In Africa these sources are found in volcanic pipes where extreme heat is responsible for cooking carbon into diamond. Other deposits include marine deposits and alluvial deposits (streambeds).

Diamonds are judged by the "four C's": carat, clarity, color, and cut. The dispersion of white light into a rainbow of colors is referred to as "fire, or scintillation". Other characteristics such as fluorescence (luminescence) and cleanliness are also important. The existence of other material decreases the clarity, and therefore the quality of the diamond. These imperfections are called inclusions. Diamonds are well known for their incredible ability to refract light, which is caused by the diamond's unique characteristics which enable it to bend and slow light as it passes through. The amount that a substance can affect light in this regard is quantified in its refractive index. When passing through a diamond light is slowed from 186,000 miles per second to about 77,000 miles per second. This is caused by the higher amounts of electrons in dense materials. The diamond's fire and scintillation is caused by the dispersion of light in which white light is is separated into all the colors of the rainbow. The refraction index can also be used to describe this phenomena. One of the more interesting characteristics of diamond is its thermal conductivity. If you touch a diamond to your lips it will feel very cold because they extract heat from your body. Some diamonds have extremely fantastic characteristics such as a rare ability to glow in the dark, or glow under a black light.

Carat weight is used to measure the mass of a diamond. One carat weighs exactly 200 milligrams (0.007 ounce), while point units are used to measure the weight of diamonds that are less than one carat. Because of the rarity of large diamonds, the value of them increases exponentially in relation to their size.