Understanding Diamond: Properties, Formation, and Identification

Diamond, a remarkable allotrope of carbon, is distinguished by its atoms’ unique arrangement in a crystalline lattice known as the diamond cubic structure. This highly stable, tetrahedral bonding configuration grants diamond its extraordinary properties. While graphite, another solid form of carbon, is thermodynamically more stable at standard room temperature and pressure, diamond persists due to a high activation energy barrier, converting to graphite only over geological timescales under these conditions.

Unique Physical and Optical Characteristics

Renowned for being the hardest known natural material, diamond exhibits an unparalleled resistance to scratching and abrasion. This attribute, coupled with its exceptionally high thermal conductivity – the highest of any natural material – makes diamond indispensable in a multitude of industrial applications. For instance, diamonds are widely utilized in cutting, grinding, and polishing tools, including drill bits, abrasive powders, and saw blades, where their extreme durability significantly enhances efficiency and lifespan. Furthermore, these superior properties are harnessed in scientific instruments like diamond anvil cells. These specialized devices can subject materials to immense pressures, mimicking conditions found deep within Earth’s mantle, allowing researchers to study material behavior under extreme environments.

The rigid atomic lattice of diamond also means it is highly resistant to contamination by impurities. However, even a minute presence of foreign atoms or structural defects can profoundly influence its appearance. For example, trace amounts of boron, substituting for carbon atoms (approximately one part per million), can impart a rare blue hue to diamonds. Nitrogen, a more common impurity, typically results in yellow coloration. Brown diamonds often owe their color to structural defects within the crystal lattice, while green diamonds are typically a result of natural radiation exposure. Other impurities or defects can lead to a captivating spectrum of colors, including purple, pink, orange, or red. Beyond color, diamond possesses a very high refractive index (leading to its remarkable brilliance) and a relatively high optical dispersion (responsible for its characteristic “fire” or the dazzling rainbow of colors it scatters).

Natural Origins of Diamonds

The vast majority of natural diamonds are ancient, with ages ranging from 1 billion to 3.5 billion years. These geological marvels originated deep within the Earth’s mantle, typically at depths between 150 and 250 kilometers (93 and 155 miles), though some rare specimens have been found originating from depths as profound as 800 kilometers (500 miles). Under the immense pressures and high temperatures prevalent in these deep-earth environments, carbon-containing fluids – such as carbonatitic or silicate melts – played a crucial role. These fluids dissolved various existing minerals and facilitated the crystallization of carbon into diamonds. Much more recently in geological terms – typically hundreds to tens of millions of years ago – these deeply formed diamonds were explosively carried to the Earth’s surface during rare, violent volcanic eruptions. They were subsequently deposited in unique igneous rock formations known as kimberlites and lamproites, which form carrot-shaped vertical pipes, making them primary sources for diamond mining.

Synthetic Counterparts and Identification

With advancing technology, diamonds can now be created in laboratories, offering alternatives to natural stones for both industrial and gem-quality applications. Synthetic diamonds are primarily grown using two methods:

• High-Pressure/High-Temperature (HPHT): This method replicates the extreme conditions found in the Earth’s mantle, using high-purity carbon sources, a metal catalyst, and intense pressure and temperature to crystallize diamond.
• Chemical Vapor Deposition (CVD): This technique involves introducing hydrocarbon gases into a vacuum chamber, where carbon atoms are gradually deposited layer by layer onto a substrate, forming a diamond crystal.

It is important to distinguish synthetic diamonds from imitation diamonds, also known as diamond simulants. Synthetic diamonds possess the identical chemical composition and crystal structure as natural diamonds. Imitation diamonds, however, are entirely different materials that merely resemble diamonds in appearance. Common examples include cubic zirconia (a crystalline form of zirconium dioxide) and moissanite (naturally occurring silicon carbide or lab-created silicon carbide). Identifying and differentiating natural, synthetic, and imitation diamonds requires specialized gemological expertise, often employing advanced optical techniques, spectroscopy, and thermal conductivity measurements, as each type exhibits distinct internal characteristics and growth patterns.

The Legendary Cullinan Diamond

The ‘Cullinan Diamond’ holds the unparalleled distinction of being the largest gem-quality rough diamond ever unearthed. Discovered on January 26, 1905, at the Premier No.2 mine (now known as the Cullinan Diamond Mine) in Cullinan, South Africa, this colossal gem weighed an astonishing 3,106.75 carats (621.35 grams or 1.37 pounds). It was named in honor of Sir Thomas Cullinan, who was the owner of the mine.

After its discovery, the Cullinan Diamond was put up for sale in London in April 1905. Despite attracting considerable international interest from potential buyers, it remained unsold for two years, primarily due to its immense value and unique nature. In a significant historical move in 1907, the government of the Transvaal Colony, then a British colony in South Africa, purchased the diamond. Prime Minister Louis Botha subsequently presented it as a goodwill gesture to King Edward VII of the United Kingdom on his 66th birthday. The responsibility of cutting this monumental gem was entrusted to the renowned firm of Joseph Asscher & Co. in Amsterdam, a task considered one of the most challenging and high-stakes in gemological history, requiring immense precision and courage to cleave the enormous stone without shattering it.

From the original Cullinan rough, a remarkable collection of nine major diamonds, 96 minor brilliants, and a number of unpolished fragments were produced. The two most prominent and historically significant stones are:

• Cullinan I (The Great Star of Africa): At an astounding 530.4 carats (106.08 grams), this pear-shaped diamond is the largest clear cut diamond in the world. It is majestically mounted in the head of the Sovereign’s Sceptre with Cross, one of the British Crown Jewels.
• Cullinan II (The Second Star of Africa): Weighing 317.4 carats (63.48 grams), this cushion-cut diamond is the second-largest stone derived from the Cullinan. It is prominently set in the front of the Imperial State Crown, another centerpiece of the British Crown Jewels.

These two magnificent diamonds, Cullinan I and Cullinan II, are among the most famous and valuable components of the British Crown Jewels, symbolizing royal power and heritage. In addition to these, seven other major diamonds from the Cullinan, totaling 208.29 carats (41.66 grams), were inherited by Queen Elizabeth II in 1953 from her grandmother, Queen Mary, and are held in her private collection. The Queen also possesses a number of the minor brilliants and some of the original unpolished fragments from this historic diamond.

Frequently Asked Questions About Diamonds

What makes diamond so hard?

Diamond's extreme hardness stems from its unique crystal structure, the diamond cubic. In this arrangement, each carbon atom is covalently bonded to four other carbon atoms in a strong, rigid, three-dimensional tetrahedral lattice. These strong carbon-carbon bonds make it incredibly difficult to break apart or scratch the material.

Are lab-grown diamonds real diamonds?

Yes, lab-grown (synthetic) diamonds are chemically, physically, and optically identical to natural diamonds. They are made of pure carbon and exhibit the same crystal structure and properties. The primary difference lies in their origin: one is formed by geological processes over billions of years, while the other is created by advanced technology in a laboratory over a few weeks or months.

How are natural diamonds brought to the Earth's surface?

Natural diamonds are brought to the Earth's surface through rare and powerful volcanic eruptions. These eruptions occur in specific geological formations called kimberlite and lamproite pipes, which act as conduits, carrying diamond-bearing rock from the deep mantle to the crust over millions of years.