Chalcedony to as prase. However, the term prase is also used to describe green quartz, and to a certain extent is a color-descriptor, rather than a rigorously defined mineral variety.) Blue-colored chalcedony is sometimes referred to as "blue chrysoprase" if the color is sufficiently rich, though it derives its color from the presence of copper and is largely unrelated to nickel-bearing chrysoprase. Heliotrope is a green variety of chalcedony, containing red inclusions of iron oxide that resemble drops of blood, giving heliotrope its alternative name of bloodstone. In a similar variety, the spots are yellow instead, known as plasma. Moss agate
Birthstone of the Second Temple, not the one described in Exodus. St. Jerome, referencing Josephus, said the Foundation Stones of the New Jerusalem (Revelation 21:19–20) would be appropriate for Christians to use. In the eighth and ninth century, religious treatises associating a particular stone with an apostle were written, so that "their name would be inscribed on the Foundation Stones, and his virtue." Practice became to keep twelve stones and wear one a month. The custom of wearing a single birthstone is only a few centuries old, though modern authorities differ on dates. Kunz places the custom in eighteenth century Poland,