Diamonds are rare and prized gemstones because they are so difficult to find. Only about 30% of diamonds are of “gem quality” and suitable for jewelry; the remaining 70% of mined diamonds, known as bort, are sold for industrial applications. To give you an idea of just how challenging it is to find diamonds, consider this excerpt from De Beers’ 2014 Diamond Insight Report:
Over the last 140 years, almost 7,000 kimberlite pipes have been sampled by geologists, about 1,000 of which have been diamondiferous. However, only about 60 of these are sufficiently rich in diamond to be economically viable. Just seven mines (Jwaneng and Orapa in Botswana, Udachny and Mir in Russia, Premier (now Cullinan) and Venetia in South Africa and Catoca in Angola) are what miners refer to as ‘Tier 1 deposits’ with more than US$20 billion worth of reserves.
Diamonds come in many colors—blue, green, yellow-orange, pink, red and even black. Colored diamonds are even more rare and valuable than their colorless counterparts. The most famous colored diamond is perhaps the Hope diamond, which is classified as a natural type IIb blue diamond. Another famous blue diamond is the Blue Moon diamond, discovered in January 2014 at the Cullinan mine in South Africa. Blue diamonds get their color from boron.
The GIA article, Study of the Blue Moon Diamond, describes the evaluation of the gem using an FT-IR spectrometer and an infrared microscope. The article explains that the Blue Moon diamond is one of few diamonds of known origin, making it valuable for study.
“FTIR spectrometry confirmed that the Blue Moon is a type IIb diamond—in other words, it lacks nitrogen observable by FTIR in the 900–1400 cm–1 region, and it contains boron… Infrared spectroscopy revealed that the amount of uncompensated boron in the diamond was 0.26 ± 0.04 ppm, consistent with measurements of several large type IIb blue diamonds previously studied. After exposure to short-wave ultraviolet light, the Blue Moon displayed orange-red phosphorescence that remained visible for up to 20 seconds. This observation was surprising, as orange-red phosphorescence is typically associated with diamonds of Indian origin, such as the Hope and the Wittelsbach-Graff…The Blue Moon underscores the fact that the phosphorescence behavior of type IIb diamonds is not tied to a specific geographical source.”
FT-IR spectroscopy can also be a useful tool for buyers and sellers to determine whether diamonds are natural and what type they are. FT-IR spectroscopy produces an infrared absorption spectrum that represents a ﬁngerprint of the sample with absorption peaks which correspond to the frequencies of vibrations between the bonds of the atoms making up the material. Because each element is a unique combination of atoms, no two compounds produce the exact same infrared spectrum.
Editor’s Note: According to EarthSky News, the next Blue Moon will be on May 21, 2016. It’s a seasonal Blue Moon, the third of four full moons between the March equinox and the June solstice of 2016.