DIAMOND   
a report created for Mineralogy by
B. M. Hosek, A Student at
Emporia State University,  Emporia, Kansas

Image taken from Unique Diamonds

What today may be considered to be the most highly prized gemstones were once just glistening pebbles amongst the many other rocks and minerals of this world. Before much was known about these glistening pebbles, man saw the intriguing external properties and fascinating features of diamond. Besides the external features, diamond contains other unique qualities that set it apart from all other minerals. Diamond originated from the Greek term adamas, which means invincible. It is an extremely hard mineral, located at the top of the hardness scale with a value of 10. Diamond is many times harder than the next hardest mineral on the scale, corundum. Although it is the hardest natural mineral, diamond has perfect cleavage in four directions. If it is struck along one of these planes, it can cleave rather easily.

Rough diamonds may have a greasy appearance or adamantine luster. Common colors are black, pale yellow, and colorless. Other colors known as "fancies" include red, pink, orange, yellow, green, blue, champagne, or brown. Diamonds first discovered in India 2000 B.C. It was not until 1725 A.D when diamonds were discovered in river gravel's in Brazil (Haggerty, 1999). The largest diamond ever found was in Africa, 1905, and weighed 3,106 carats.

Image taken from DeBeers

The Hope diamond, located in the Smithsonian Institution Museum in Washington, D.C. Image taken from DeBeers

Formation and Geologic Occurrence In the past, diamonds have been found in meteorites that have fallen to the earth. Some believe that diamond was actually contained in the meteorite while others believe that diamond forms from metamorphic processes that occur when the meteorite strikes the Earth. Although diamonds are found in alluvial deposits, their source is kimberlite or lamporite rock, in intrusive bodies termed pipes (Bruton). The name kimberlite comes from the diamond mines in Kimberly, South Africa. These kimberlite pipes are igneous intrusions and referred to as diamond pipes. Kimberlite is a common component of old continental cratons. The composition of kimberlite may include peridotite which is rich in magnesium (olivine), pyroxene, garnet, and chromium spinel (Haggerty). Other productive diamond locations include India, Russia, South America, South Africa, and Australia. Diamond is believed to originate at depths of more than 180 km (Haggerty).

Image taken from Haggerty

Image taken from Diamond Carbon

Crystallography Diamond belongs to the isometric crystal system (4/m Bar 3 2/m), and has perfect four directional cleavage. Its luster is adamantine or greasy. The brilliance of diamond is related to its high dispersion (.044) and high refractive index (2.42), which is a measure of how far it can bend light. The fracture along the cleaved plane of diamond is conchoidal, and the tenacity is brittle.

Diamond is most commonly found in the octahedron {111} form. Other forms include cubic or hexahedron {100}, tetrahedron, and rhombic dodecahedron {100} (The Mineral Diamond). Convex faces with the octahedron and hexoctahedron forms are common characteristics instead of a flat plane (Bruton). Twinning is common, usually producing a flattened stone (see below) (Klein and Hurlbut).

Image taken from (Bruton)

Image taken from (Bruton)

Internal Structure
The composition of diamond is carbon. Each carbon atom joins with four other carbon atoms to form covalent bonds. The close packing of carbon atoms are what give it its high specific gravity of 3.52, and the high melting points (3,820 degrees Kelvin) (Bruton). Graphite is a polymorph of diamond and contains a different structure and atomic arrangement. The internal crystal structure of diamond is a face-centered, cubic unit cell, lattice (Klein & Hurlburt). The internal crystal structure is related to the bonding of carbon atoms.

Physical Properties
Besides its high hardness, diamond has high specific gravity 3.52. It is also a semi-conductor. Nitrogen is the most common trace element found in diamond, which can consist up to 0.23%. Other trace elements include aluminum, boron, silicon, calcium, iron, copper, magnesium, barium, chromium, silver, titanium, strontium, sodium and lead.

Grading

Grading diamonds is based on internal and external properties. From rough diamonds to the final gem product, similar processes are used. Flaws in rough diamonds may be removed before the diamond is cut. Although imperfections may still be removed on a cut stone with laser drilling. Diamond grading is based on carat weight, cut, clarity, and color, or the 4 C's. These factors affect the sparkle and value of a diamond.

Image taken from Matlins and Bonanno, 1998.

Image scanned from Matlins and Bonanno, 1998

Carat The carat portion of the 4 C's refers to the weight. A diamond's weight is given in carats (ct), a universal standard, where one carat equals 200 milligrams. Carat is sometimes confused with the size of the diamond. Poorly cut diamonds can appear to be larger or of a higher carat weight when simply matched with a diagram, such as the one to the left (click on the image to bring up a larger version, although this is not true to life because of varying monitor resolutions). One carat is also equal to 100 points, a system jewelers sometimes use (e.g., 50 points equals a one-half carat stone). The weight of a diamond can greatly affect the value, however bigger is not always better.

Cut The cut of a diamond is the most important of the 4 C's. Grading cut depends upon specific angles and symmetry. A poorly cut diamond may eliminate the importance of the clarity, color and carat. Flaws in a cut may include a shallow or deep (heavy) cut, a shallow or deep crown, a sloping table, or a combination. The girdle is also important. A diamond can have bad symmetry with an out-of-round girdle, wavy girdle, or a girdle that is too thick or thin.

Image taken from DeBeers

Image taken from Matlins and Bonanno, 1998

There are many different ways diamond can be cut. The round brilliant cut is the most common, nearly seventy five percent of all cuts. Other common cuts include marquise, oval, emerald, princess, pear-shaped, trillion, and heart shaped. The lower left image shows an emerald cut diamond, and the lower right chart show a list of cuts or see other cuts cuts, scanned from Schuman, 1997.