Kyanite specimens from this border area between the towns of Montejo de la Sierra (Madrid) and El Cardoso de la Sierra (Guadalajara) have been the subject of discussion for a long time. In the area there are different deposits from which they can belong. In this specimen, the Baltasar Sánchez label is preserved, indicating the specimen from El Zahurdón, in Montejo de la Sierra. In the area we have toponyms such as Corrales de Zahurdón. Peter Seroka, to whom this piece belonged, indicates already in 1973 the presence of kyanite about 1000 m from the milestone of Km 3 of the road from Montejo to El Cardoso (Miguel Calvo's Mines and Minerals of Spain), see discussion in this link.
This specimen shows a group of kyanite crystals on a matrix with quartz, with an intense color. It is accompanied by the two mentioned labels. Very interesting...