I got this rock yesterday from a friend who was unsure of what it was. I think it’s carnelian but what’s interesting I what’s inside!! Any ideas what it is?
This is quite a nice agate. The entire rock is likely made of various forms of quartz from crypto-crystalline (microscopic crystals) to macro-crystalline (visible crystals) maybe. Agates form when silica waters enter a void in the host rock under conditions where the silica can start to precipitate out. This is why agates are often found in old lava flow, beds, with some porosity (voids), in the presence of rocks with a higher silica component such as Rhyolite, there also has to be a water component sometime in the geologic past. The bottom section likely formed first with the silica enriched water precipitating out larger crystals, followed by the upper outer section where the Carnelian color and the finer crystals (closer to the crypto-crystalline) precipitated out on the surface of the void. The inner banding under the carnelian is likely slower precipitating silica forming larger crystals and banding. Agates like this can often have a hollow center with larger crystals, but in this case it looks like it filled the center.
Overall, the inner section is likely the same, or similar, type of material as is found in the bottom. Essentially it is all quartz with the colors from various impurities and with varying crystal size creating some of the banding effect.
The Wikipedia article on agates give a reasonable explanation but the images are not varied enough to show the many forms, Wiki - Agates