These buttons were actually blown and were hollow. The early ones had clear glass loop shanks; the later (probably after 1900) had metal shanks on metal plates. Color and sometimes bits of tinsel were blown into some of the hollow buttons. Most common are those that imitate pearl color, but there were a few made in pastel colors as well as in deep red, gray, and black; some have been found with cotton or a waxlike substance inside. Blown glass buttons were usually ball-shaped, but a few have been found faceted or oval. They range in size 3/8 to 3/4.
The above information is from the book Collector's Encyclopedia of Buttons by Sally Luscomb.
The piece of delicate material looks to of been cut off a garment to save the buttons..notice the back, the buttons were strung in such a way as to remove them easily before cleaning the garment.
Below this are some of my blown glass..all have the glass shanks.
Click on image to enlarge
