A cumdach or book shrine is an elaborate ornamented box or case used as a reliquary to enshrine books regarded as relics of the saints who had used them in Early Medieval Ireland. They are normally later than the book they contain, often by several centuries, typically the book comes from the heroic age of Irish monasticism before 800, and the surviving cumdachs date from after 1000, although it is clear the form dates from considerably earlier. Several were then considerably reworked in the Gothic period. The usual form is a design based on a cross on the main face, with use of large gems of rock crystal or other semi-precious stones, leaving the spaces between the arms of the cross for more varied decoration. Several were carried on a chain or cord, often suspended round the neck, which by placing them next to the heart was believed to bring spiritual and perhaps medical benefits (the same was done with the St Cuthbert Gospel in a leather bag in medieval Durham). They were also used to witness contracts. Many had hereditary lay keepers from among the chiefly families who had formed links with monasteries. Most surviving examples are now in the National Museum of Ireland ("NMI").