Only one piece has two little posts in back that look to me like where you would glue a pin, to make it a brooch - this is just the one, otherwise they are all smooth-backed. And these "posts" are really quite stubby and shouldn't interfere if you were sewing these into something. I'll show them in photos.

I don't know what they're for.  They're theatrical. My mind goes to creating costumes e.g. "princely robes."  They're obviously cheap, nothing precious, but they have a loud garishness appropriate for the stage.

They're neat.  I guess you could do anything with them.

They measure between 2 and a quarter and 2 and a half inches in width and length.

They are vintage, so there will be some surface scuffing where they are "faceted" - but nothing glaring.

One little oddball "sapphire" that is heart-shaped in a butterfly frame.