The Herkimer Diamond is naturally formed as a double terminated quartz crystal but does come in many different sizes. There are many grades of stone available. Some are very clear while others do have inclusions and visible dark markings. Regardless of the clarity, they all work the same however the clear stones are much more highly sought after and are usually much more expensive.
In crystal healing, the Herkimer Diamond is often used at the Crown Chakra either by itself or in addition to another stone (usually the quartz crystals point). When used with the Quartz Crystal, the effects of the Herkimer Diamond are enhanced quite a bit and it increases the flow of energy going through the Crown Chakra.
These Herkimer Diamonds are considered large size, each one will vary in size and clarity but will measure approximately 1/2" long.
3rd Eye, Crown