Diamonds have special societal significance, but where do they come from?
Widely regarded as the most precious of gemstones, diamonds are actually pure carbon. What differentiates diamonds from graphite, an opaque, black mineral that is also pure carbon, is the unique process that forms diamonds. This process produces a strong chemical structure that gives diamonds their durability and versatility in industrial and cosmetic applications (1).
Carbon atoms are most stable when they form four bonds (2). In a diamond, each carbon is covalently bonded to four other carbons, creating a three-dimensional tetrahedral pattern, as shown in Figure A (1). This repeated tetrahedral structure facilitates the stability of its carbons, and thus makes diamond one of the strongest, most durable natural materials (3,4).
Diamonds are created within magma in the Earth’s mantle and require compression of ~50,000 times atmospheric pressure, or about 23,000 times car tire pressure, about 150 kilometers under Earth’s surface, and high temperatures greater than 1000 degrees celsius to form (5). Most diamonds were created over 1 billion years ago, and some are as old as 3.3 billion years (4). Once a diamond has formed deep under Earth’s surface, it must be carried toward the top through a kimberlite pipe (4). Kimberlite pipes are cooled small eruptions of magma from 100-300 kilometers beneath Earth’s surface that carry diamond-containing magma upward (5). Many diamonds are found by mining kimberlite pipes, although rivers or glaciers can erode the kimberlite pipes and transport diamonds to other locations (4). There are far more diamonds remaining in the magma of Earth’s mantle than there are at Earth’s surface.
The large diamonds often used for jewelry are only a fraction of all of the diamonds formed in the Earth. The difficulty of finding high-quality, sizable diamonds in nature, as well as the ethical issues related to poor mining conditions, such as low pay and long hours, has motivated scientists to develop methods to form synthetic diamonds. In 1960, researchers created the first synthetic diamond (6). Now, they are widely used as they are cheaper, larger, faster to produce, and of higher quality than natural diamonds.
The process of creating synthetic diamonds takes only a few days and uses relatively small machines (6). One way to make synthetic diamonds is to mimic the temperature and pressure conditions of the inner Earth (6). A small diamond chip is placed in graphite, and the extreme conditions created by the machines cause a diamond to form (7). Alternatively, small machines create a plasma, a 100,000ºC mixture of ions and electrons, that contains carbon and hydrogen atoms. The plasma causes the carbon atoms to precipitate onto a small “seed” diamond called a diamondoid, which contains fewer than 50 atoms of carbon (8). Once the process begins, carbon atoms will gather around the diamondoid, resulting in a large, high-quality diamond (8).
Synthetic diamonds are often used to provide the hard edge for tools such as saw blades among other industrial applications. Some companies are now also creating synthetic diamonds for use as gemstones (6). Scientists can change the color of a synthetic diamond by intentionally introducing impurities, such as boron and nitrogen, during production or by exposing the diamond to radiation post-production (4). Synthetic diamonds have the same chemical composition as natural diamonds, and most consider the synthetic to be virtually indistinguishable from natural diamonds, though they fetch much lower prices when sold as gemstones. Synthetic diamonds are especially gaining favor among those concerned about the social costs of mining diamonds.
This is the tetrahedral geometry of the carbon in a diamond. The carbon atom numbered 2 in this diagram has four bonds coming off of it, and each bond is angled about 109.5 degrees from the other bonds connected to that carbon atom. (link)
This diagram illustrates the formation of kimberlite pipes millions of years ago and the discovery of diamonds today. (link)
1 14.4A: Graphite and Diamond – Structure and Properties. (2015, June 20). Chemistry LibreTexts. https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Map%3A_Inorganic_Chemistry_(Housecroft)/14%3A_The_Group_14_Elements/14.04%3A_Allotropes_of_Carbon/14.4A%3A_Graphite_and_Diamond_-_Structure_and_Properties
2 1.7: Common Bonding Patterns for Organic Chemistry. (2019, February 13). Chemistry LibreTexts. https://chem.libretexts.org/Courses/Sacramento_City_College/SCC%3A_Chem_420_-_Organic_Chemistry_I/Text/01%3A_Introduction_and_Review/1.07%3A_Common_Bonding_Patterns_for_Organic_Chemistry
3 Giant covalent structures of carbon – Bonding, structure and properties of materials – OCR 21st Century – GCSE Chemistry (Single Science) Revision – OCR 21st Century – BBC Bitesize. (2022). BBC Bitesize. https://www.bbc.co.uk/bitesize/guides/z8kgqhv/revision/2
4 Encyclopædia Britannica. (n.d.). Diamond. Britannica Academic. Retrieved October 9, 2022, from https://academic.eb.com/levels/collegiate/article/diamond/30264
5 How Diamonds are Formed | Cape Town Diamond Museum. (2015). Cape Town Diamond Museum. https://www.capetowndiamondmuseum.org/about-diamonds/formation-of-diamonds/
6 Yarnell, A. (2014). THE MANY FACETS OF MAN-MADE DIAMONDS. Chemical & Engineering News; American Chemical Society. https://cen.acs.org/articles/82/i5/FACETS-MAN-MADE-DIAMONDS.html
7 Lab-Grown Diamond Production Methods – International Gem Society. (2021, October 5). International Gem Society. https://www.gemsociety.org/article/lab-grown-diamond-production-methods/
8 Meriame Berboucha. (2018, August 22). This Is How Synthetic Diamonds Grow. Forbes. https://www.forbes.com/sites/meriameberboucha/2018/08/22/this-is-how-synthetic-diamonds-grow/?sh=4b5c127769d7