Virginia Diamonds

After the Big Bang 13.7 billion years ago, hydrogen atoms fused inside stars and formed elements up to iron. When a star exploded as a supernova, a gas cloud was created - and as the cloud cooled:1

[t]iny crystallites of pure carbon – diamond and graphite – were probably the first minerals in the universe.

Meteorites still bring microscopic and occasionally larger black diamonds (carbonados) to earth from those star clouds, but all diamonds found in Virginia were formed here on earth. Only four diamonds have been found within the current boundaries of Virginia, and a fifth was found just downstream of the border of Giles County. That diamond probably eroded out of Virginia bedrock, before washing downstream into West Virginia. As of 1997, four of the diamonds had been sold and could no longer be traced, but the diamond found southeast of Richlands in Tazewell County was still owned by the family on whose farm it was discovered.2

Those five diamonds crystalized not on the surface, but near the crust-mantle boundary 75-100 miles underground. Most diamonds are ancient, forming around 500 million-1 billion years ago. That was long before land plants developed, so Virginia's diamonds were not created by compressing coal under great pressure. Much later, perhaps during the Mesozoic Era when dinosaurs lived in Virginia, the diamonds were shoved up to the surface. The diamonds rose from the mantle through narrow volcanic "pipes," moving as much as 100 miles in less than 2 days. The climb to the surface took the diamonds through changing zones of heat and pressure, but was completed so quickly (perhaps as fast as 30 miles per hour) that the crystals were not oxidized into graphite/carbon dioxide and destroyed.3

We know where the Virginia diamonds were found (the "Dewey" or "Manchester Diamond" was found at Ninth and Perry streets, in what is now the City of Richmond) but exactly where those diamonds originally erupted onto the surface of the earth is unknown. Two potential sources within Virginia are a kimberlite pipe in Rockbridge County (near Mt. Horeb Church) and a mica peridotite dike in Warren County - assuming any site in Virginia is the source. The diamonds found in Virginia could have been transported through erosion from out-of-state source materials in Kentucky, Arkansas, the Wyoming/Colorado border, or in Canada.4

<
approximate locations of Virginia diamonds and potential source rocks
approximate locations of Virginia diamond finds and potential source rocks:
- Front Royal (Warren County) peridotite: potential source for Vaucluse Mine, Orange County (1) and Whitehall Mine, Spottsylvania County (3) diamonds
- Mount Herob (Rockbridge County) kimberlite: potential source for Dewey Diamond (2)
- those sources, or perhaps Kentucky kimberlites, Arkansas pyroclastic lamproite tuff, or Wyoming/Canadian kimberlite pipes could be the sources for the Tazewell County (4) and "Punch Jones" (5) diamonds
Source: derived from Virginia Division of Mineral Resources, Diamonds

If the Warren County mica peridotite dike was the source for the Orange and Spottsylvania county diamonds, then the crystals may have eroded out of the dike and been transported east 100-150 million years ago when the Potomac Formation was deposited during the Cretaceous Period. Erosion and transport must have occurred long ago, before the development of the Shenandoah Valley. Once the Blue Ridge was exhumed by erosion and the Shenandoah River pirated the streams that had flowed to the east, then any diamonds in that dike would be washed downhill towards the Potomac River rather than deposited in the Rappahannock River watershed.

Other potential sources for diamonds in Virginia may exist in Triassic basins or elsewhere in Virginia, where the distinctive volcanic features associated with diamond delivery from the mantle have not been recognized yet. The "large igneous province" created during the eruption of the Catoctin basalts 550 million years ago may have included plumes from the core–mantle boundary that brought diamonds to the surface. More recent Eocence epoch eruptions at Trimble Knob (Highland County) and Mole Hill (Rockingham County) are diatremes, too young to appear to be considered normal source rocks for diamonds... but who knows for sure?

The five different finders recognized they had unusually-glittering pebbles, did not immediately identify them as diamonds. (The brilliant sparkle of a diamond in a modern ring has accentuated by the style in which the raw stone was cut.) The "Punch" Jones Diamond, found in 1928 along Rich Creek just downstream from the Virginia border (Giles County), caught the eye of a father and his 12-year old son as they played horseshoes in the family yard. The rock was stored in a cigar box for 14 years before the son asked a geologist at Virginia Tech to assess what is now considered to be the largest diamond discovered in/near Virginia.5 An article in the 1859 Harper's New Monthly Magazine noted about the Dewey Diamond:6

The great marvel in this Virginia diamond is not that it was found, but that it was retained by the finder; for were it dropped among the pebbles at Cape May or Newport, it would have been among the last to be elected as being "so like a diamond."

All Virginia diamonds were found in unconsolidated sediments, rather than in their original geologic context, so they may have been transported significant distances. In 1942, the state geologist stated that the Dewey Diamond:7

must either have been brought down the James River and deposited with some of its sediments, or have been introduced accidentally by man into these stream deposits.

The raw stone later cut into the Dewey Diamond was found in six feet of clay, and no one claimed they had somehow lost that distinctive "pebble" found by a laborer levelling the road, so it was probably transported down the James and deposited on the old floodplain. However, there is one more possibility regarding how a diamond ended up at the Fall Line, far from any known kimberlite or lamprophyre source rocks today. In 1910, the Richmond Times-Dispatch speculated that the stone had been recognized as unique by by Native Americans and buried with an Indian chief, and the road clearing had excavated the grave and revealed the special stone.8

Links

References

1. Robert M. Hazen, The Story of Earth: The First 4.5 Billion Years, From Stardust to Living Planet Viking, 2012, p.12
2. Palmer C. Sweet, "Diamonds in Virginia," Virginia Minerals, Virginia Division of Mineral Resources, Vol. 42 No. 4 (November 1996), p.1, http://www.dmme.virginia.gov/dmr3/dmrpdfs/vamin/vamin_vol42_no04.pdf (last checked May 22, 2012)
3. David A. D. Evans, "Earth science: Proposal with a ring of diamonds," Nature vol. 466, pp.326–327 (15 July 2010), doi:10.1038/466326a, http://www.nature.com/nature/journal/v466/n7304/full/466326a.html; Richard A. Lovett, "How Diamond-Studded Magma Rises From Earth's Depths," National Geographic, January 19, 2012, http://news.nationalgeographic.com/news/2012/01/120119-diamonds-gems-earth-magma-carbonate-rocks-science/; Cate Lineberry, "Diamonds Unearthed," Smithsonian, December 2006, http://www.smithsonianmag.com/science-nature/diamond.html"> (last checked May 22, 2012)
4. Palmer C. Sweet, "Diamonds in Virginia," pp.1-2
5. Palmer C. Sweet, "Diamonds in Virginia," p.3
6. Harper's New Monthly Magazine Volume 0019 Issue 112 (September 1859) / Volume 19, Issue: 112, September 1859, pp. 466-481, http://digital.library.cornell.edu/cgi/t/text/text-idx?c=harp;idno=harp0019-4 (last checked May 22, 2012)
7. "What Do You Wish To Know," Richmond Times-Dispatch, October 18, 1942, p.53
8. "Daily Queries and Answers," Times-Dispatch (Richmond), November 9, 1910


Minerals of Viginia
Volcanoes in Virginia
Rocks and Ridges - The Geology of Virginia
Virginia Places