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This Wiki article is a little confusing. First it writes of impact breccia containing shocked quartz, then later it speaks of tiny “diamonds.” So which is it quartz, or diamonds formed from carbon? Also, this made me think of the fine quartz crystals found in Herkimer, NY which are called Herkimer “diamonds.” Now I am wondering about the origin of those little “gems.” Too late to search now. Must sleep.

Herkimer diamonds are not actually diamonds, but are double-terminated quartz crystals of exceptional clarity (water-clear) discovered within exposed outcrops of dolostone in and around Herkimer County, New York and the Mohawk River Valley.[1] The “diamond” in their name is due to both their clarity and natural faceting - crystals possess double termination points and 18 total facets (six on each point, six around the center).

The geologic history of these crystals began about 500 million years ago in a shallow sea which was receiving sediments from the ancient Adirondack Mountains to the north. The calcium and magnesium carbonate sediments accumulated and lithified to form the dolostone bedrock exposed as the Little Falls Dolostone today.[3] While buried, cavities were formed by acidic waters forming the vugs in which the quartz crystals formed. While the dolostone unit is Cambrian in age the quartz within the vugs is interpreted to have formed during the Carboniferous Period.[4] Waxy organic material, silicon dioxide and pyrite (iron sulfide) was present as minor constituents of rock made of dolomite and calcite. As sediment buried the rock and temperatures rose, crystals grew in the cavities very slowly, resulting in quartz crystals of exceptional clarity. Inclusions can be found in these crystals that provide clues to the origins of the Herkimer diamonds. Found within the inclusions are solids, liquids (salt water or petroleum), gases (most often carbon dioxide), two- and three-phase inclusions, and negative (uniaxial) crystals. Anthraconite is the most common solid inclusion.

So the Herkimer diamonds are probably not the same as the Nördlingen diamonds. The German diamonds were formed via impact concussion, whereas the Herkimer diamonds were grown quartz crystals.

So sayeth wiki.