From Vitruvius Book VII, Chapter 13
1. I shall now speak of purple, which, above all other colours, has a delightful effect, not less from its rarity than from its excellence. It is procured from the marine shell which yields the scarlet dye, and possesses qualities not less extraordinary than those of any of the body whatever. It does not in all places where it is found possess the same quality of colour; but varies in that respect according to the sun’s course.
2. Thus, that which is obtained in Pontus and in Galatia, from the nearness of those countries to the north, is brown; in those between the south and the west, it is pale; that which is found in the equinoctial regions, east and west, is of a violet hue; lastly, that which comes from southern countries possesses a red quality: the red sort is also found in the island of Rhodes, and other places near the equator.
3. After the shells are gathered they are broken into small pieces with iron bars; from the blows of which, the purple dye oozes out like tears, and is drained into mortars and ground. It is called ostrum, because extracted from marine shells. Inasmuch as this colour, from its saltness, soon dries, it is prepared for use with honey.
At St. Peter’s Basilica
“The excavations revealed the remains of shrines of different periods at different levels, from Clement VIII (1594) to Callixtus II (1123) and Gregory I (590-604), built over an aedicula containing fragments of bones that were folded in a tissue with gold decorations, tinted with the precious murex purple. Although it could not be determined with certainty that the bones were those of Peter, the rare vestments suggested a burial of great importance.”
Murex purple, also known as Tyrian purple
“Tyrian purple (Greek, πορφύρα, porphyra, Latin: purpura), also known as royal purple, imperial purple or imperial dye, is a purple-red natural dye, which is extracted from sea snails, and which was first produced by the ancient Phoenicians. This dye was greatly prized in antiquity because it did not fade, rather it became brighter and more intense with weathering and sunlight.”