Herkimer diamonds are double terminated quartz crystals found in cavities of dolostone host rock in New York State, USA.
Herkimer diamonds are pretty famous. You will find lots of websites talking about these beautiful crystals. Many of these sites are about astrology and New Age but there is usuful stuff too.
The fact that these crystals are actually not diamonds doesn’t make them any less interesting. What makes these crystals so unique? It is the fact that they have nearly perfect crystal faces on every side. This is really remarkable. Quartz crystals grown in cavities are not that rare but they are attached to the wall of the cavity on one end. So they have only one end with perfect crystal faces. Herkimer diamonds had to grow so that they had minimal contact with the host rock. I really don’t know why and how they managed to do that but it should be obvious that crystals grown that way must be exceedingly rare. The extreme clarity of the crystals makes them very appealing as well.
The host rock of the crystals is approximately 500 million year old dolostone. This does not mean that the quartz crystals found inside the rock are that old. They are definitely younger. First there was lime mud on the bottom of the shallow sea. This mud was buried and compacted until it became hard rock which we know as limestone. This limestone had its composition altered by the magnesium bearing fluids which slowly turned it into dolostone. Dissolution vugs or cracks had to form somehow and only then quartz crystals had a chance to start growing very slowly out of the silica bearing water that was moving through the rock formation.
You may also want to take a look at the article about the Pecos diamonds which are also double terminated quartz crystals but they grew in gypsum deposit.
These Herkimer diamonds (double terminated quartz crystals) were collected 40 years ago. The width of the view is approximately 10 mm.
I thank Bill Beiriger for sending me these beautiful crystals.