Jet is of vegetable origin, being fossil wood, a variety of, but very much harder than ordinary coal, and capable of taking a very high polish. The finest is the well-known Whitby jet which was first discovered by the monks of the historical Abbey before the Reformation brought its ruin. Jet is also found on the coasts of the Baltic, and in the Middle Ages was known as Black Amber, being worn as a prophylactic against epilepsy and fits, and to prevent strangulation of the womb; taken internally, powdered in wine, it was considered good for the toothache, and mixed with beeswax was used for tumours. It was also well known and used by all ancient nations, the Greeks dedicating it to Cybele, the goddess of all things necessary to life produced by the earth, wearing it for her favours and especially for protection to travellers by sea and land. Boetius de Boot recommends it as a specific against nightmare, witchcraft, and melancholy apprehension.

Neither the Ruby, Malachite, Black Onyx, nor Jet should be worn by the Libra or Aries type, or indeed by any who have Saturn afflicted in their birth map.