Dietrich revises the date of the Vaucluse find to 1847 in his later Minerals of Virginia (published in 1990), and notes that Manchester is now part of Richmond. The Dewey Diamond is also known as the Manchester Diamond.
The "reportedly found" phrase is significant. A diamond may have fallen from some traveller's pocket in Manchester, rather than be derived from local geology. Virginia has some geologic features west of Danville that might be source rocks for diamonds, but otherwise Virginia is not a likely source for such minerals. [There's probably a fascinating story behind the people who claimed to have found each diamond... and what might have really happened before the diamonds were "found."]
Where would you look for diamonds in Virginia? Dr. Ralph Moberly, Professor in the Department of Geology and Geophysics at the University of Hawaii, answered this on the Ask-An-Earth-Scientist page. He reports that diamonds are typically found in diatremes, narrow (1square km) "pipes" of volcanic material that erupted with supersonic speed from deep (100km) in the earth. These pipes are known as "kimberlites" after the discovery of these geologic features in Kimberly in southern Africa in 1871, but are also found in Australia, Asia, Brazil - and in Arkansas and Wyoming.
Most geologic changes in rocks are slow, gradual, hand-me-the-clicker-let's-watch-another- channel events. But supersonic flow of rock may occur in more than just kimberlite pipes. The basaltic dikes in the Triassic basins (Danville, Richmond, Culpeper...) may have been created after the sandstone cracked slightly and a froth of superheated rock was pressured into them. A slow oozing of molten basalt into the narrow cracks would have resulted in swift cooling of the rock, blocking the remaining rock from expanding into a small crack. Lava flows show this constant damming and rerouting of the flow, especially in the formation of pillow basalts. Today the dikes are often mined for road construction material - and then the basalt flows very slowly, truck by truck, in first gear down the highway...