The Herkimer diamond is not actually a diamond but instead a special kind of double-terminated quartz, known for being very clear and having, ideally, 18 facets. (You sometimes get triple- or quadruple-terminated Herkimers, which throw off that number.) Technically, these can occur anywhere, but to be a Herkimer diamond, they must be found in one of the mines occurring in Herkimer County, New York, where they are unusually common.
This small stone with remarkable clarity is believed by some to have attunement properties that allow it to connect to the soul and "stimulate metaphysical abilities." People have used it for such purposes as interplanar travel, dreamwork, past life recall, spiritual transformation. It is said to be "programmable" and to be a high-vibration stone.
Possible correspondences: americana, astral, dream,