harebell

A perennial flowering plant, Campanula rotundifolia, native to the Northern Hemisphere, with blue, bell-like flowers.

How Snow-drops cold, and blue-eyed Harebells blend / Their tender tears, as o'er the stream they bend […].

Her most striking feature, then as now, was her eyes: both large and expressive, but while one was the brown of a fresh hazelnut, the other was pale bluey-violet, the colour of harebells.

foxglove

I wonder whether the witches would be wearing on their fingers their ornaments of foxglove blossoms, often called witches' bells or harebells, sometimes known as witches' thimbles.