The wonders of calcium.

Today, a marine shell sitting on an alabaster dish. Best viewed large.

In modern usage alabaster is a fine-grained variety of gypsum a soft mineral composed of calcium sulphate dihydrate. In Europe, the centre of the alabaster industry is in Tuscany, Italy, where the alabaster occurs in masses embedded in limestone. This little dish was fashioned from alabaster mined near to the hilltop town of Volterra, a major centre of the Italian alabaster industry.

Whereas the alabaster dish was fashioned by human hands, the complex shell sitting on it was made by an Argonaut, a muscular little pelagic octopus. Like alabaster, the shell is also made from calcium, but this time in the form of calcium carbonate deposited on a framework of protein.

Only female Argonauts grow a shell, which they use as a case to protect their eggs, which they lay inside the shell in long threads. The female argonaut lives in the entrance of her shell and guards the eggs, until the young hatch.

You can see a photograph of a living Argonaut, and read more about them here.