Herkimer, double terminated rock crystal. Called Herkimer diamond originally because of its high clarity and strong reflections of light, they are usually double terminated. The name Herkimer comes from the type locality Herkimer in NY State, USA. These are from there.
(Herkimer is more widely used now, also for other small double-terminated rock crystals, which come from other places and do not need to be as "dazzled")
On the wild side, the ones for sale are shown. Most have attachment marks / natural irregular indentation, on one long side. Any of the pictures show them with the likely " backside " up. If there are any small black inclusions, it is probably anthracite.
Approx. 9.5mm to 10.5mm long