Tuffeau is a marine sedimentary rock which is found in the Loire Valley of France.
The Loire Valley formed the floor of a vast sea 90 million years ago. Over the millennia, sediment from the sea floor, comprising fossilized living organisms and sand particles, became compressed to form what is now known as Tuffeau stone.
Mining of Tuffeau stone for construction reached its peak in the 15th century, and the mining techniques used to extract the valued stone created a vast network of troglodyte or troglodytic caves in the Loire Valley, France, through which flows the great Loire River. The caves have been used as dwellings in the past, partly due to the practicality that the indoor temperature is remarkably constant from summer to winter. Today, many of the larger caves are used for growing a wide variety of mushrooms, which are transported daily to the markets in Paris. The Loire is the largest supplier of mushrooms to Paris.
Today, a number of older dwellings, from the magnificent Château d'Ussé and Château de la Motte d'Usseau to the many modest worker's cottages (Longères), are still standing having been built from large blocks of this wonderful material. A particularly beautiful building constructed of tuffeau is the Château de Beaulieu which stands on the banks of the Loire River just outside the town of Saumur.
Tuffeau has a very low density compared with many other rocks; for example, it is about half as dense as granite and, indeed, is only about 10 to 20% heavier than water. In fact, ebony wood is almost as dense as tuffeau stone! Also, tuffeau has an amazing porosity of up to 50%, whereas that of granite is only about 1%. The porosity of tuffeau can easily be demonstrated by projecting water onto a tuffeau wall (by hose or other means) and observing its immediate absorption by the wall; with granite, there would be, of course, clear 'splash back'. Given tuffeau's porosity it is not surprising that the compressive strength of the stone is a factor of ten to twenty times less than that of granite.
It's a beautiful, but fragile, stone.