Diamonds may not be as rare as once believed, but this finding in a new Johns Hopkins University research report won't mean deep discounts at local jewelry stores. "Diamond formation in the deep Earth, the very deep Earth, may be a more common process than we thought," said Johns Hopkins geochemist Dimitri A. Sverjensky, whose article co-written with doctoral student Fang Huang appears today in the online journal Nature Communications. The report says the results 'constitute a new quantitative theory of diamond formation,' but that does not mean it will be easier to find gem-quality diamonds and bring them to market.

For one thing, the prevalence of diamonds near the Earth's surface -- where they can be mined -- still depends on relatively rare volcanic magma eruptions that raise them from the depths where they form. For another, the diamonds being considered in these studies are not necessarily the stuff of engagement rings, unless the recipient is equipped with a microscope. Most are only a few microns across and are not visible to the unaided eye. 

Using a chemical model, Sverjensky and Huang found that these precious stones could be born in a natural chemical reaction that is simpler than the two main processes that up to now have been understood to produce diamonds. Specifically, their model -- yet to be tested with actual materials -- shows that diamonds can form with an increase in acidity during interaction between water and rock. 

The common understanding up to now has been that diamonds are formed in the movement of fluid by the oxidation of methane or the chemical reduction of carbon dioxide. Oxidation results in a higher oxidation state, or a gain of electrons. Reduction means a lower oxidation state, and collectively the two are known as 'redox' reactions. 

"It was always hard to explain why the redox reactions took place," said Sverjensky, a professor in the Morton K. Blaustein Department of Earth and Planetary Sciences in the university's Krieger School of Arts and Sciences. The reactions require different types of fluids to be moving through the rocks encountering environments with different oxidation states.

The new research showed that water could produce diamonds as its pH falls naturally - that is, as it becomes more acidic -- while moving from one type of rock to another, Sverjensky said.