In the magico-medical tradition of Europe and the Near East, the aetites (singular in Latin) or aetite (anglicized) is a stone used to promote childbirth. It is also called an eagle-stone, aquiline, or aquilaeus. The stone is said to prevent spontaneous abortion and premature delivery, while shortening labor and parturition for a full-term birth.
From Theophrastus onwards, the belief is also recorded that the stone had the ability to "give birth" to other stones, based on the crystals found within. This fed into the belief that at least some minerals could be gendered into male and female forms.
A nodule consisting of a hard shell of hydrated oxide of iron, within which the yellow oxide becomes progressively softer toward the center, which is sometimes quite empty.— 
The American Geological Institute defines the eaglestone as "a concretionary nodule of clay ironstone about the size of a walnut that the ancients believed an eagle takes to her nest to facilitate egg-laying."
According to Dioscorides (5.160), the aetite should be fastened to the left arm to protect the fetus; at the time of birth, it should be moved to the hip area to ease delivery. He also recommends them for the treatment of epilepsy, and says that when mixed with meat they will "betray a thief". Pliny describes four types of aetites in his Natural History and outlines their magico-medical use:
Attached to pregnant women or to cattle, in the skins of animals that have been sacrificed, these stones act as a preventive of abortion, care being taken not to remove them till the moment of parturition; for otherwise procidence of the uterus is the result. If, on the other hand, they are not removed at the moment when parturition is about to ensue, that operation of Nature cannot be effected.
Jewish medical practice
Jewish women used birthing stones, and the Talmud refers to the "preserving stone," worn as an amulet even during Shabbat to prevent miscarriage. Although medieval sources point to the eagle-stone, the identification is not certain. Rabbis in medieval France and Germany, and a Polish talmudist in the 16th century, describe the stone as hollow, with a smaller stone inside: "the stone within a stone represented a fetus in the womb." One medieval French source says that the stone "is pierced through the middle, and is round, about as large and heavy as a medium sized egg, glassy in appearance, and is to be found in the fields."
Medicine to 1700
The aetite, to be carried by pregnant women on their right side, is mentioned by Ruberto Bernardi in his 1364 book of popular medical lore. The Italian Renaissance philosopher Ficino ascribes the aetite's ability to ease childbirth to the astrological influences of the planet Venus and the Moon. In 1494, Isabella d'Este, the marchesa of Mantua, expressed her confidence in the power of these stones. The aetite appears in a Spanish work on natural magic by Hernando Castrillo, first published in 1636. Alvaro Alonso Barba's work on metallurgy (Madrid, 1640) touts the efficacy of the aetites, advising that the stone be tied to the left arm to prevent spontaneous abortion, and to the right arm for the opposite effect. The work was widely reviewed, reprinted and translated.
The 1660 book Occult Physick said the aetite
is white and round like a Tennis-ball, and hath a stone that shaketh within it. Being worn it delivereth women in their extremity, but at any other time it is not to be used by them that are with Child. It is good to be worn for the Stone … Feavers and Plague. It doth also dissolve the knobs of the Kings Evil (i.e., scrofula), being bound to the place grieved.— 
Aetite, along with bloodstone (hematite), was the subject of a 1665 book by J.L. Bausch, municipal physician of Schweinfurt and founder of the Academy of the Curious as to Nature. Bausch, however, cautions that empty promises of the stone's powers exceed the limits of both medicine and nature. Thomas Browne affirmed the stone's application to obstetrics in his Pseudodoxia Epidemica (1672), but doubted the story about eagles.
- Harris, Nichola Erin, The idea of lapidary medicine, 2009, Rutgers University, Ph.D. dissertation (book forthcoming), available online as PDF
- Stol, Marten. Birth in Babylonia and the Bible. Styx Publications, 2000. Limited preview online.
- Thorndike, Lynn. A History of Magic and Experimental Science.
- The eagle-stone is defined as "the common name of the aetite" by Thomas Wright, Dictionary of Obsolete and Provincial English (London, 1886), p. 414 online.
- Sarah Iles Johnston, Restless Dead: Encounters Between the Living and the Dead in Ancient Greece (University of California Press, 1999), p. 167 online.
- Harris, 47-48
- Jacqueline Marie Musacchio, The Art and Ritual of Childbirth in Renaissance Italy (Yale University Press, 1999), p. 140 online.
- Albert H. Fay, A Glossary of the Mining and Mineral Industry (Washington Government Printing Office, 1920), p. 18 online.
- American Geological Institute, Dictionary of Mining, Mineral, and Related Terms (Birkhäuser, 1992, 2nd ed. 2003), p. 179 online.
- Marten Stol, Birth in Babylonia and the Bible (Styx, 2000), p. 50 online.
- Harris, 53
- The full passage may be read in Bostock's English translation online.
- Pliny, Natural History 36.39.
- Sarah Iles Johnston, Restless Dead (University of California Press, 1999), p. 167.
- Talmud Bavli Shabbat 66b.
- Stol, Birth in Babylonia and the Bible, p. 51; Michele Klein, A Time to Be Born: Customs and Folklore of Jewish Birth (Jewish Publication Society, 2000), p. 38 online.
- Lynn Thorndike, A History of Magic and Experimental Science, p. 334.
- Full title Arte de los metales en que se enseña el verdadero beneficio de los de oro y plata por açogue. El modo de fundir los todos y como se han de refinar y apartar unos de ostros. Thorndike, pp. 258 and 260.
- William Williams "Philosophus", Occult Physick (London, 1660), p. 94, quoted in Thorndike, p. 321 online.
- The word "hematite" derives from Greek hema-, "blood"; but in modern usage, bloodstone may refer to heliotrope.
- Full title in Latin Scediasmata bina curiosa de lapide haematite et aetite ad mentem Academiae Naturae Curiosorum congesta. Lynn Thorndike, A History of Magic and Experimental Science, p. 262 online.
- Thomas Browne, Pseudodoxia Epidemica (1672) II. v. 9, as cited by Stol, Birth in Babylonia and the Bible, p. 50, note 15.
- Thorndike's multi-volume classic work is available widely online, where it is often not identified accurately by volume and year.