Wave worn pebbles on the beaches of my childhood, hiking mountains and studying rocks and landscape as a geologist all contribute to my deep appreciation of stone. I have two favourite stones for sculpture, limestone and marble.

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Limestone and marble have essentially the same composition but are very different as sculptural materials. Looking at the formation of these two materials helps us to understand their differing performance. Limestone is a sedimentary rock, formed of shell, bone and coral fragments and lime grains that have been cemented together. It varies considerably in material properties depending on the origin and age. It is typically creamy to grey in colour with dull surfaces. It may be uniform in texture or chock full of fossils; it may be soft enough to cut with a knife or so hard that a hammer and chisel are needed to break it.

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Marble is formed when limestone is subjected to heat and pressure. During this metamorphic process the mineral calcite which forms all of the shell and lime fragments becomes crystalline. The original structure of any fossils is lost as the rock is heated and squeezed and the crystals of calcite grow into an interlocking matrix. Impurities in the original limestone will affect the colour and may result in interesting veins of colour through the marble.

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It is the crystalline nature of marble that makes it such a marvellous sculptural medium. The stone needs to be worked with sharp chisels and rasps but the surface can be polished to a silky lustre. The capacity of calcite to reflect light both from the surface of a crystal and from planes within the crystal gives a particular life to a marble sculpture.

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