Purple dye murex (Bolinus brandaris) edible if boiled long enough but are now endangered so should be left alone.
They played a major role in ancient Mediterranean history, however, as the source of the much sought after royal or Tyrian purple (from Tyre, in modern Lebanon) that dyed the cloaks of the Roman Caesars, the sails of Cleopatra and the robes of the Byzantine emperors and empresses. Unlike other dyes, the purple didn't fade, but only grew brighter when exposed to the elements. Known as πορφύρα (porphyra) in Greek, the colour was so precious that a child born to an imperial couple was called porphyrogenitos, 'born in the purple'.
Detail from Saffron Gathers fresco, Akrotiri, Santorini, pre 1630 BC
The Minoans were making the dye as early as 2000 BC. Each snail was only good for a drop of two of essential purple mucus (or 'milk', which they excrete was a defense mechanism), so the snails, fished in spring and fall, had to be kept alive until the dyers had enough mucus to colour their cloth. The Minoan technique of just crushing the snails against the cloth was perfected by the Phoenicians, who according to archaeologists mixed the 'milk' with sea water and vinegar or potash in waist-high vats, exposed to sunlight to deepen the dye from yellow to crimson (the tone preferred by the Roman emperors) and then boiled it with hyacinth flowers to give a deeper mauve or purple tint preferred by the Byzantines.
Both Justinian and Theodora are lavishly clad in the most prized shade of purple ('blackish clotted blood') in their famous portraits in San Vitale in Ravenna.
However they did it, the process was very smelly.
Often heaps of Bolinus brandaris's dye making murex cousins (Hexaplex trunculus) were found in same spots and were (probably) used to make a different tint, including the indigo blue of the ancient Jewish Tekhelet dye, used in the garments of the High Priest, Tabernacle tapestries and prayer shawls. The method was lost when the Temple was destroyed, but that according to a chemical analysis of the ancient fragments that have survived, the colour was derived from snails. The blue on the Israeli flag recalls it.
The snails were once so abundant in Greece that dye works have been several Greek islands, including an impressive Roman-era installation on the now-uninhabited islet of Koufonissi off the southeast coast of Crete. Unlike the Phoenicians and Romans who killed the snails, the indigenous Mixtec people of Mexico, who made a similar dye from a similar species (Plicopurpura pansa), 'milked' the snails simply by tickling or blowing on them and put them back in the sea, which is why the snails are still abundant and the Mixtec are still making the dye today.
Images by: 16:9clue, Sharon Mollerus, PD art