Fossilized coral. These are the skeletal remains of what used to be very colorful marine invertebrae. This was the primary building material for the historic churches in the Philippines. The exoskeleton is calcium carbonate (If you recall your Earth Science, limestone is formed from CaCO3). I use this particular coral for a neat ashtray.

Canon EOS M10
88mm + 16mm macro xtension tube
f6. 3, 1/80s, ISO200