1. Altunkaynak S, Özdilek Y (2006). Timing and nature of postcollisional volcanism in western Anatolia and geodynamic implications. Geological Society of America, Special Paper 409, 321 – 351.
2. Arzoğulları U (2007). Dereyalak köyü (Eskişehir) çevresindeki agat ve opal oluşumlarının jeolojisi ve ekonomik önemi.The Master of Science (MS) Programs, Çanakkale Onsekiz Mart University, Çanakkale, Turkey (in Turkish with English abstract).
3. Brauner K, Preisinger A (1956) Struktur und Entstehung des sepioliths. Tschermaks Mineralogische und Petrographische Mitteilungen 6:120–140 (in German)
4. Çalık A (2017). "Geology and mineralogy of Belkavak opal occurrences - Kütahya, NW Turkey”,70 th Geological Congress of Turkey, Ankara, Turkey, (132), pp. 120-121(in Turkish with English abstract).
5. Çalık A, Arzoğulları U (2009). Geology of the agate and opal – bearing formations in the Dereyalak village (Eskişehir), Turkey, Special Issue - Proceedings of the 16th Meeting of the Association of European Geological Societies, 9 - 13 July 2009, Cluj - Napoca, Romania.