Chalk under a microscope......if you ever wonder why i like using chalks to make pigments just look!

​Chalk rock (calcium carbonate), a pure form of limestone formed in warm, tropical seas about 100 million years ago in the Cretaceous Period when Dinosaurs ruled the Earth!
Microscopic marine algae, called coccoliths, lived in the ancient sea.  Their shells were made of calcite.  As the algae died, their bodies sunk to the sea floor thus chalk sediment was deposited, so Chalk rock could be described as a "biological graveyard!"
Over millions of years layers of chalk sediment were deposits caused compaction of loose sediment into solid chalk rock.