An illuminated manuscript is an illustrated book, and was one of the most important forms of art in medieval Europe (and elsewhere, including the Islamic Empire). During the early Middle Ages, European illuminated manuscripts were an important part of Christian missionary activities, and Christian monasteries were at the heart of manuscript production. Specially trained monks, called scribes, wrote text on two different types of animal skin, vellum and parchment, as paper was not common until the fifteenth century. Medieval illuminators supplemented the text with colorful decoration, often designing large initials and full-color illustrations that took up an entire page. Illuminated manuscripts were very labor-intensive and expensive. The ink alone was worth as much as a semi-precious stone, and sometimes gold leaf was used in the decoration. Manuscripts were sometimes protected with expensive, jewel-encrusted covers, such as the cover of the Lindau Gospels.