Herkimer Diamonds - What are they?

Nowadays more and more people are choosing silver over gold and more affordable gems over diamonds. One name that popped up in a search I was doing was "Herkimer Diamonds". At a price of less than $50 for a necklace, it seems quite affordable. So, what is a Herkimer Diamond?

By Ra'ike (see also: de:Benutzer:Ra'ike) - Eigenes Werk (own work), mit freundlicher Unterstützung durch GemArt-Unikate (Marianne Kempf, Hatten), CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=5838542

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. 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). Because the first discovery sites were in the village of Middleville and in the city of Little Falls, respectively, the crystal is also known as a Middleville diamond or a Little Falls diamond.

Herkimer diamonds became widely recognized after workmen discovered them in large quantities while cutting into the Mohawk River Valley dolostone in the late 18th century. Geologists discovered exposed dolostone in Herkimer County outcroppings and began mining there, leading to the "Herkimer diamond" moniker. Double-point quartz crystals may be found in sites around the world, but only those mined in Herkimer County can be given this name.

A typical Herkimer diamond has 18 facets: six upper, middle, and lower facets.  They are also known for being very bright and incredibly clear, although they can also come in other rare formation and colors.

For example, these Raw Stone Engagement Rings, Rose Gold Plated Herkimer Diamond from https://www.etsy.com/shop/thefoxandstone

or

This Herkimer Diamond Necklace in Silver or Gold from https://www.etsy.com/shop/AbizaJewelry