The great Italian goldsmith and sculptor, Benvenuto Cellini (1500–1574), relates that when a youth he often shot cranes with his arquebuse, and that in several instances he found in their entrails not only fine turquoises, but also fragments of the so-called plasma-emerald and even occasionally small pearls. This serves to indicate that the pretty exterior of such objects exerted an influence upon these birds in some degree analogous to the impressions aroused in mankind on viewing them.[35] In seventeenth century Denmark there seems to have been no lack of “magic stones,” for it is related that one day as King Christian II was strolling along the beach, he picked up a shining pebble by the aid of which he could render himself invisible at will. Similar power was said to exist in stones that could be found in ant-hills if hot water were thrown onto them on St. Walpurgis Day, or St. Hans’ Day. The Danes of the time also shared in the belief that the stone from the lapwing preserved from illness and sorrow as did the “swallow’s-stone” as well.[36] It has frequently been maintained that the source of pebbles could be broadly determined by their form and surface; for example, well-rounded specimens of fairly uniform size would be classed as marine pebbles, while river-pebbles would be subangular and usually flat; pebbles of glacial origin, on the other hand, would have faceted, rounded edges, their surfaces being polished and striated. However, although these rules might hold good in many cases, careful observation has demonstrated that pebbles of all these supposedly distinct types can be found among those of marine, fluviatile, or lacustrine origin. This is explicable by the fact that while the constant, unhindered action of sea or river would probably produce pebbles of distinct type, the local conditions often interfere with this. For instance, on a low sea-coast, with weak wave-action, pebbles frequently became buried in the sands, thus retaining their form practically unchanged, and even where the waves are stronger, so that the pebbles are more or less constantly exposed to their force, it must be borne in mind that some of these coast pebbles have been swept down by rivers, or have already been affected by glacial action. In these cases the force of the waves will indeed modify the form, but along the lines of that already produced by the earlier agencies. Broadly stated, those that were round or oval would generally remain so, rectangular fragments might have their angles worn away and become elliptical, while flat fragments would not exhibit any notable change in shape.[37]