"Heliotrope is a stone of such extraordinary virtue that the bearer of it is effectually concealed from the sight of all present." Dante in his "Inferno," xxiv.

92, further alludes to it: "Amid this dread exuberance of woe Ran naked spirits winged with horrid fear, Nor hope had they of crevice where to hide, Or heliotrope to charm them out of view." In the same way the agate was said to render a person invisible, and to turn the swords of foes against themselves.[19] The Swiss peasants affirm that the Ascension Day wreaths of the amaranth make the wearer invisible, and in the Tyrol the mistletoe is credited with this property.
But some plants, as we have already pointed out, were credited with the magic property of revealing the presence of witches, and of exposing them engaged in the pursuit of plying their nefarious calling.

In this respect the St.John's wort was in great request, and hence it was extensively worn as an amulet, especially in Germany on St.John's Eve, a time when not only witches by common report peopled the air, but evil spirits wandered about on no friendly errand.

Thus the Italian name of "devil-chaser," from the circumstance of its scaring away the workers of darkness, by bringing their hidden deeds to light.

This, moreover, accounts for the custom so prevalent in most European countries of decorating doorways and windows with its blossoms on St.John's Eve.