This looks like some anatomical specimen or a cut of meat shown in cross section, but relax- it's a rock. Alex Sample of Sample's Agate Gem and Mineral Shop posted this beautiful rhodochrosite stalactite. Rhodochrosite is a crystalline mineral that blushes in many shades of pink and red, but can be other colors. Rhodochrosite is the national gemstone of Argentina, and the state mineral of Colorado. Stalactites form when water dripping from the ceiling of a cave leaves mineral deposits over time. You can see from this cross section how a cluster of five small stalactites were eventually covered by a larger layer of rhodochrosite.
When this was posted at reddit, the title was "Here’s a picture of the Museum Quality Rhodochrosite Stalactite Specimen Without the light shining through it". That's the Instagram caption, but without context, everyone wanted to see the image with the light.