Dactylomancy or dactyliomancy is a form of divination using a suspended ring.
Derived from reek daktylios, a finger-ring.
In some traditions of dactylomancy, a ring is suspended like a pendulum above a surface that is marked with letters or symbols. The direction of the swing indicates which symbols are to be consulted, or which letter are to be formed into a message, in answer to a specific question.
Another tradition follows the same pattern as Séance table-rapping. In it a ring is suspended from a tumbler so that it may touch the sides if swung and a code is agreed upon (e.g., 1 for yes, 2 for no). A question is then posed and the number of the times that the ring strikes the side of the tumbler is interpreted as being an answer.
In yet another method, the ring was dropped into a bowl of water, its position at the bottom determining the prediction or the response to a formulated question. Sometimes the inside bottom of the bowl contained a special pattern and/or symbols imprinted, to aid in the prognostication.
A more complex form of dactylomancy was practiced in Europe during the Middle Ages in which a ring was suspended above a circular table marked with the symbols of the zodiac. 78 metal discs inscribed with a letter of the alphabet (three discs per letter) were then placed on the table and the thread holding the ring was burnt. The letters that the ring rolled across and the one on which it halted were then consulted to form the answer to the question being divined. In this tradition, the metal (gold, silver, copper, iron, or lead) that from which the ring was made of was determined by the day of the week.
Monday – Silver, to represent the moon
Tuesday – Iron, to represent Mars
Wednesday – Tin/lead, to represent Mercury
Thursday – Tin, to represent Jupiter
Friday – Copper, to represent Venus
Saturday – Lead, to represent Saturn
Sunday – Gold, to represent the sun 
According to Ammianus Marcellinus, Dactyliomancy was used to find Valens' successor, and the name Theodosius was correctly indicated. Solemn services of a religious character accompanied this mode of divination.
The rite was religiously performed, the diviner, entirely clothed in white linen, and with his head shaven, held in his hand a piece of vervain, which is well known protection against evil spirits; the ring also was consecrated.
By the Middle Ages, the names of the Three Wise Men – Caspar, Melchior, and Balthasar – were popular as inscriptions. Later, the custom was applied to wedding rings, which had the words “Love and Obey” engraved on the inside, but such a positive prediction could be nullified if the bride crossed her fingers.
Jacob, P. L. (1878) Science and Literature in the Middle Ages, and at the Period of the Renaissance, Bickers and Son
Spence, Lewis (2003), Encyclopedia of Occultism and Parapsychology, Kessinger Publishing, ISBN 0766128156
Cunningham, Scott (2003), Divination for Beginners: Reading the Past, Present and Future, Llewellyn Worldwide, ISBN 0738703842