Complex sentence: There are certain facts that speak for the heating/bleaching process having taken place at a high altitude or in some sort of ancient vacuum chamber; otherwise the smoothness and the absence of cracks in the agate cannot be explained.
Split into simpler sentences: There are certain facts that speak for the heating/bleaching process taken place at a high altitude or in some sort of ancient vacuum chamber. Otherwise the smoothness and the absence of cracks in the agate cannot be explained.