Totally baffled

Whilst sorting out stuff around the house we found two flat white paper envelopes, totally sealed, clean and undamaged, with no markings on them. Each envelope was 15 cm by 21.7 cm. They looked as though they might contain some kind of large wound dressing. There was no easy way to open them so I cut one end off one of the envelopes.

Inside the envelope (which was lined with plastic film) was a rectangle of thin plastic with, attached to it, a thin, fairly fragile, grey layer of something dry. Embedded in this thin grey layer is a sprinkling of dark grey flecks, some, at least, of which are sparkly.

Any ideas what this thing is?

The plastic and the grey coating are each, perhaps, 0.1 mm thick.

The envelope

The contents of the envelope.

Close up of the fragile grey film.