Geode-like rock but it's empty inside. Why does this happen?
Geodes form where there is a gas bubble or some other sort of cavity in volcanic or sedimentary rocks. I once found scores of them eroding out of a sea cliff in Oman, and they looked very similar to yours. I can tell that your geode was formed in sedimentary rock like mine were. What happens is that over millions of years, water seeps in, sometimes bringing in sand and clay particles which line the cavity when it occurs in sedimentary rock. This mineral rich water, sometimes from hydrothermal sources, seeps into the cavity and precipitates various minerals, mostly silicon dioxide, around the sides of the hollow. This usually results in a lining of quartz crystals, but can produce amethyst or opal, both of which are composed of silicon dioxide. Very rarely, opalized fossils are found inside geodes. As for your empty geode, I presume that either the necessary seepage of mineral-laden water never happened, or it never had time to happen. The process takes millions of years.