Sep
19
You Can’t Get a Diamond from a Lump of Coal
No, that title isn’t meant to be a metaphor. I’m talking about actual diamonds and coal.
As has been well-established, diamond is an incredibly hard, crystalline form of carbon that requires extreme heat and pressure to form. However, contrary to what you may have heard, read, been taught, or watched on TV, diamonds do not form from coal. Yeah, that surprised me, too. (So much for that childhood image of Superman squeezing a lump of coal into a diamond for Lois Lane.) But, that’s what Hobart M. King says. (He isn’t the only one, of course.) King is a PhD geologist and gemologist, a respected educator in the field, and the manager and publisher of Geology.com since 2005. Here is King’s explanation in condensed form…
The problem with the “diamonds from coal” story is two-fold: 1) most naturally-formed diamonds have been dated to many millions of years earlier than the source material of coal (i.e., the first land plants); 2) coal seams are horizontal units of sedimentary rock, whereas diamonds are found in vertical units of igneous rock (i.e., kimberlite and lamproite).
Virtually all natural diamonds found at or near Earth’s surface are theorized to have resulted from the following four processes (and virtually all of those from the first):
1) Formed In The Mantle — Roughly 90+ miles below the Earth’s surface, usually beneath stable interiors of continental plates, there are “diamond stability zones” that exhibit both the requisite heat and pressure extremes. Carbon trapped there eons ago sometimes forms into diamonds and on rare occasions is forced up to the surface by volcanic eruption through kimberlite and lamproite pipes.
2) Formed In Subduction Zones — Tiny (but commercially unusable) diamonds form when carbonate rocks like dolomite, limestone, or marble are moved deep into the mantle — say 50-400 miles beneath the surface — via subduction and later moved (in diamond form) back up to the surface. Mineral inclusions have shown evidence that subducted seawater and/or oceanic crust are involved.
3) Formed At Impact Sites — When an asteroid or meteor strikes the Earth, more than enough heat and pressure are produced than is needed for diamond formation. Supporting evidence comes from the tiny diamonds (i.e., ranging from under a millimeter to 13 millimeters) found at various impact sites. Again, the source carbon is most likely from dolomite, limestone, marble, or other rock with carbon in it.
4) Formed In Space — The tiniest diamonds (i.e., only a few nanometers in length, so way too small for commercial use) have been found in meteorites themselves. They are believed to have been formed during high-speed collisions in space. No coal involved, though, and the carbon source would most likely have been extraterrestrial in origin. (Ooooooohh!)
To get a fuller explanation of all this (along with helpful graphics), you should read King’s relatively short, lay-friendly article: “How Do Diamonds Form?” Geology.com is a treasure trove of information about rocks, minerals, gemstones, volcanoes, etc. Check it out!
P.S. Another article — actually, a podcast w/ transcription — on the subject can be found from “The Naked Scientists”.
