No. 95: Sep-Oct 1994
How can something as beautiful, pure, and crystalline as a diamond be made from dead, disgusting bacteria? In truth, all diamonds are full of impurities and curious microscopic structures. (See: "Diamonds Are an Anomalist's Best Friend" in SF#92.)
The main constituent of diamonds is carbon, but even chemically pure carbon is contaminated in a sense. The contaminant is light carbon; that is, C12 , which is an isotope used preferentially by living organisms. Some diamonds, it is found, contain anomalously large fractions of C12, which suggests they have an organic origin. Some diamonds also contain sulfide inclusions that have sulphur-isotope ratios also symptomatic of a biological origin.
The specific diamonds suspected to have an organic origin are the so-called "eclogitic" diamonds. These diamonds may have obtained their carbon and impurities from bacterial communities that once lived around hydrothermal vents that existed along ancient mid-ocean ridges. Subsequent metamorphism (heat and pressure) turned the masses of bacteria into eclogitic diamonds. So, those sparklers of yours may just be clumps of billion-year-old bacterial corpses!
(Nisbet, E.G., et al; "Can Diamonds Be Dead Bacteria?" Nature, 367:694, 1994.)
Definition. Eclogites comprise a class of metamorphic rocks formed at extremely high temperatures and pressures.