Half of a geode!

This rock appears to be a type of geode, specifically an agate geode. The outer part is typically composed of a layer of chalcedony, which is a form of silica, often found in volcanic and metamorphic rocks. The inside, as seen in the photo, shows botryoidal formations, which are grape-like clusters, commonly formed by the deposition of minerals from groundwater within the cavities of the rock. The white and translucent nature of the interior suggests it could be lined with quartz or agate, which often forms in such structures. These formations are prized for their beauty and are often cut and polished for use in jewelry or as decorative pieces.