Identifying decorative stones, such as those used in the Courtauld Gallery’s pietre dure frame, may be one of those instances where scientific investigation is actually inappropriate. Whereas geologists use destructive analytical techniques to accurately identify rocks and minerals from the field – smashing them open or finely slicing them to view under a microscope – these methods are not an option when considering valuable and historically important objects from a museum collection.
Moreover, even some non-intrusive techniques are not particularly useful for decorative stone identification. Most analytical techniques, such as XRF and Raman spectroscopy, provide information on either crystal structure or chemical composition. However, many materials that look very different may have essentially the same chemistry. For example, glass, quartz, amethyst, agate and jasper are all composed of silica (SiO2). Therefore, chemical techniques can tell us what a stone is, but can rarely tell us where it came from.
As a result, decorative stones are normally identified by a combination of observation, connoisseurship and comparison with other stones of known provenance. An excellent resource for identifying decorative stones is the extensive Corsi collection. Composed of exactly 1000 polished stone slabs of uniform shape and size (145 x 73 x 40 mm), the collection was established in Rome in the first quarter of the 19th century and is now housed in the Oxford University Museum of Natural History.