These glass-like jewels are the crystallised and outgrown wings of a fairy. Once a fairy grows their adult wings, the young ones are shed and become crystallized from the years of use with pixie dust to fly.
Occasionally you may find a family of fairies who decorate their windows with shed baby wings as a decorative memento of childhood.
Note for Mortals:
Crystallized fairy wings are approximately three inches in size and come in a black organza drawstring pouch for storage, stones are made from agate. Colour and size selected at random per order. An ideal gem to add to a collection and display on a shelf.