“Christus crystallus”: rock crystal, theology and materiality in the medieval West.

This chapter focuses on the interactions between theoretical knowledge and material choices in the arts of the Middle Ages in the West.1 It considers a widely employed artistic medium – rock crystal – and it explores the multiple symbolic meanings associated with this stone as well as examining its employment specifically in relation to a cross reliquary preserved in Borghorst, Germany. Due to its physical properties of hardness and transparency and its widely accepted associations with congealed water, rock crystal was approached in the medieval West as a metaphor for a variety of theological notions: angelic natures, the Incarnation of Christ, the sacrament of baptism and the miraculous conversion of souls hardened by sin. Knowledge of these theological implications assists in explaining the popularity of rock crystal in the liturgical arts of the Middle Ages and allows a better understanding of its specific artistic meanings, particularly in the manufacture of reliquaries. In addition, the enigmatic and ambivalent nature of rock crystal, which encompassed the opposing qualities of liquid and solid bodies, provided visual substantiation to otherwise elusive notions concerning the ontological status of created beings and their participation in the divine. Thus, it offers us a privileged standpoint from which to analyse medieval attitudes towards matter and its connection with the spiritual world.