Portrait of Steno in the
Church of San Lorenzo, Florence.
The idea that crystals are formed from repeating unit cells grew from his principle of the constancy of the angle. That is, the angle between pairs of corresponding faces is the same for any crystal of a particular substance, regardless of the way it has grown or what shape it has. He was a fascinating and complex character, and, as Carlo Mielli mentioned in the IUCr Newsletter, his remains now lie in the Church of San Lorenzo in Florence. We paid a visit during this year's IUCr meeting, fortunately after getting a useful tip from Rudolf Allmann that it was pointless paying and looking in the main body of the church, where the tombs of the Medici are. His memorial is in the part reserved for private prayer - and is consequently free of charge!
But first a word about him. He was a true Renaissance man, and it instructive to see how many web-sites mention him. One, in fact, hails him as a "creation scientist" - rather normal, one would have thought, in the seventeenth century! He was trained in medicine, both in Denmark and the Netherlands, and rose to be Royal Anatomist in Copenhagen in 1667. Among other things, he gave his name to the excretory duct of the parotid gland (ductus stenonianus).
On the side, he was a geologist, and it was his observation of quartz crystals that led to his famous principle. Perhaps as part of observing and tiring of the conflict in doctrine between Lutheran Denmark and Calvinist Holland, he gave up both, converted to Roman Catholicism, and moved to Italy. He became a priest, and in 1677 the titular Bishop of "Titiopolis". For most of the rest of his life, he was an apostolic delegate in northern Germany, and seems to have stopped writing either medicine or science, but producing philosophical works of a very eirenic nature for their time.