Botryoidal Geode!

This rock appears to be a geode filled with chalcedony or agate. Here's a detailed explanation:

Geode: Geodes are hollow, vaguely spherical rocks, often formed in volcanic and sedimentary rocks. They have a thin outer layer and are typically lined with crystals inside. The one in the image has been cut open to reveal its interior.
Chalcedony: This is a cryptocrystalline form of silica, composed of very fine intergrowths of quartz and moganite. Chalcedony can appear in various colors and forms, including agate, which has banding or layers.
Agate: If the chalcedony within the geode shows banding or layers, it could be classified as agate. Agate typically has more distinct color bands, which might not be very clear in this specimen due to the type of formation or the cut.
Formation: Geodes form when gas bubbles or volcanic cavities get filled with mineral-rich water. Over time, the minerals precipitate out of the water, forming layers or crystals inside the cavity. In this case, the minerals appear to have formed botryoidal (grape-like) structures, which is common in chalcedony.
Appearance: The white to light gray, rounded, botryoidal formations inside the geode suggest chalcedony. The outer shell seems to be a mix of quartz and possibly some iron staining, giving it the brownish hues.