Pilgrims are given a stone to carry with them to make lime for the construction of the Apostle’s basilica
LSJ
Authorship of the text of Book Five of the Jacobus, the so-called Pilgrim’s Guide is attributed to Pope Calixtus II and a certain Aimery and it’s colophon tells us that it was largely composed at the abbey of Cluny in Burgundy.
It goes without saying that such medieval attributions of authorship and provenance need to be treated with scepticism. The career of the Burgundian Calixtus, as a former monk of Cluny and eventual holder of the Papal throne did little to qualify him for the familiarity with the topography of northern Spain demonstrated by the text. Furthermore, Calixtus’ death in 1124 precludes him as the definitive author.
The dates 1137, which marked the death King Louis VI of France and is referred to in the text and 1173, when the first known copy was completed, provide the time frame for the compilation of the Jacobus. According to the text, the cathedral of Compostela had already been under construction for sixty-three years at the time of King Louis’ death and building was still ongoing.
This close proximity of the text to the phenomenon it described is further evidenced by the mention of the continuing practice at Triacestela, 
of pilgrims gathering stones which they carried to Castaneda, eighty kilometres further on. These went into the making of the lime used in the construction of the cathedral, giving the modern reader a vivid sense of the contemporaneity of the text.
It seems evident that the author had a firm grasp of the fine detail of the pilgrimage road, especially when one considers that Chapter III entitled “Of the names of towns on this road”, lists a total of fifty seven place names along the route. On this basis, an origin for the text of the Pilgrim’s Guide within the scriptorium of the cathedral of Santiago seems likely.
Biblio: J. Passini – Sur le chemin de Saint Jacques. Cahiers de Saint Michel de Cuxa XXX 2000 pp.75-83, W. Melczer – The Pilgrims’ Guide to Santiago de Compostela