Boškovski, B., S. Bogoevski, G. Ruseska, and K. Atkovska. “High Temperature Crystallization Process into Opalized Tuff”. Geologica Macedonica, Vol. 29, no. 2, Dec. 2015, pp. pp. 209-213, https://js.ugd.edu.mk/index.php/GEOLMAC/article/view/1230.