Diamonds are made of carbon atoms arranged in a crystalline structure. Diamonds form deep underground under intense heat and pressure conditions. The carbon source comes from deposits of organic materials that were part of ancient lifeforms. Diamonds can form in several types of rock formations and volcanic pipes. The unique properties of diamonds, including hardness, brilliance and crystal shape, are determined by their molecular structure.
Diamond Formation
Diamonds are formed from carbon atoms subjected to immense heat and pressure deep underground. The required conditions occur between 80-120 miles (130-200 km) below the earth’s surface, where temperatures can reach over 2,000°F (1,100°C). At this depth, the pressure can be over 45,000 times greater than surface pressure.
Over billions of years, natural forces move carbon deposits downward where they are compressed and heated into diamond crystals. The carbon source comes from the remains of ancient living organisms. Plankton fossils and plant matter accumulate on ocean floors and river deltas forming sedimentary deposits. Over long time periods, tectonic plate movements subduct these carbon-rich sediments deep into the mantle.
Diamonds typically form in one of two geologic structures:
Cratonic Roots
Cratons are ancient stable blocks of continental crust. The lower section of cratonic rock extends deep into the mantle like roots. Diamonds form within these cratonic roots at depths between 150-200 km where conditions are right for crystallization. The cratonic roots provide a steady source of carbon to fuel diamond growth.
Volcanic Pipes
Volcanic pipes occur in vertical cylindrical shafts formed by volcanic activity. As magma rises quickly from the mantle, it can carry diamonds and other deep-earth minerals upwards. The magma act as a fast elevator that allows diamonds to reach the surface in the solidified volcanic rock. Kimberlite and lamproite pipes are the main diamond-bearing pipe structures.
Diamond Mineral Properties
Diamonds have a unique combination of physical and chemical properties that make them valuable as gemstones and industrial abrasives.
Chemical Composition
Diamonds are crystalline allotropes of the element carbon. The carbon atoms bond tetrahedrally to four other carbons generating a rigid three-dimensional lattice structure. The tight crystal lattice gives diamonds their hardness and strength.
Hardness
Diamonds are the hardest known natural mineral with a rating of 10 on the Mohs hardness scale. The strong carbon bonds make it resistant to scratching and cutting. Diamond’s hardness allows it to serve as an abrasive for cutting tools.
Brilliance
Diamonds have an extremely high refractive index which gives them remarkable optical dispersion and fire. Light passing through a diamond is split into flashes of spectral colors. Well-cut diamonds exhibit exceptional brilliance and sparkle.
Crystal Habit
Diamonds naturally form as octahedral crystals, although cube, dodecahedron and other shapes can occur. Cutting facets on diamonds enhances the optical effects.
Diamond Grading
Diamonds are evaluated and graded according to the 4Cs – cut, clarity, color and carat weight. These characteristics determine the quality and value of individual diamonds.
Cut
Cut refers to the angles and proportions of the facets cut onto the diamond rough. Well-cut diamonds have more brilliance and fire. Diamonds are cut according to different shapes such as round brilliant, princess, oval and more.
Clarity
Natural diamonds often contain birthmarks and imperfections known as inclusions. Diamonds with fewer inclusions and blemishes have greater clarity. Diamonds are graded from Flawless (FL) to Included (I3) based on visibility of inclusions under 10x magnification.
Color
Diamond color grading ranges from D (colorless) to Z (yellow/brown). Colorless and near-colorless grades are most desirable. Trace elements like nitrogen can cause detectable yellow or brown hues. Fancy colored diamonds in shades of yellow, pink or blue are rare and valuable.
Carat
Carat weight measures the mass of a diamond. One carat equals 200 milligrams. Larger diamonds are exponentially rarer and more valuable. High quality one-carat diamonds are treasured in fine jewelry.
Famous Diamonds
| Name | Carat Weight | Description |
|---|---|---|
| Hope Diamond | 45.52 | Famous historic blue diamond with rare natural deep blue color. |
| Cullinan Diamond | 3,106 | Largest rough gem diamond ever found. Cut into 9 major diamonds for British Crown Jewels. |
| Koh-i-Noor | 105.6 | Oval cut white diamond with controversial ownership history. Part of British Crown Jewels. |
| Great Star of Africa | 530.2 | Largest cut colorless diamond. Set in British Sovereign’s Scepter. |
Diamond Industry
Global diamond mining produces over 130 million carats annually. Diamonds are mined on every continent except Antarctica. The top production regions are Africa, Russia, Australia and Canada. After mining, rough diamonds are sent to trading centers for sorting and valuation. Most gem-quality diamonds are cut and polished in India, China and Thailand. The diamond industry pipeline connects miners, rough diamond dealers, manufacturers, jewelers and consumers.
Diamond Jewelry
Diamond jewelry has been coveted for centuries as symbols of love, commitment and status. Diamonds are extensively used in engagement rings, wedding bands, necklaces, earrings and other fine jewelry. Modern technology has enabled the production of affordable, mass-market diamond jewelry using small diamonds set in gold or silver. Diamond simulants made with cubic zirconia and moissanite mimic diamond’s shine at a fraction of the cost.
Industrial-Grade Diamonds
While gem diamonds command the highest prices, 80% of mined diamonds are unsuitable for jewelry and used for industrial purposes. These diamonds may be flawed, irregularly shaped or small crystals. Industrial applications rely on diamond’s hardness as an abrasive and cutting tool. Uses include stone cutting, drilling, grinding, polishing and machining. Diamond demand for heavy-duty industrial wear parts is linked to growth in construction, mining and oil/gas extraction.
Synthetic Diamonds
In addition to natural diamonds formed in the earth, synthetic diamonds can be produced under controlled laboratory conditions through High Pressure High Temperature (HPHT) or Chemical Vapor Deposition (CVD) processes. Synthetic diamond is chemically identical to natural diamond, but tends to have higher purity and uniformity. Initially used for industrial needs, labs can now grow synthetic diamonds for jewelry at more affordable prices.
Conclusion
Diamonds form underground due to a unique combination of heat, pressure and carbon-rich geochemistry over billions of years. The strong crystalline structure of diamonds leads to superlative physical properties including extreme hardness and optical dispersion. Diamonds are highly valued as gemstones and industrial abrasives. Diamond’s rarity, beauty and symbolism cement its importance in jewelry as a representation of love and commitment. Advances in synthetic diamond production are providing ethically sourced, economical sources to complement natural diamond’s position in the gem and industrial markets.
