The supposition of the end of the paintbrush being used isn't based only on the birefrigent material but also on some cellulose being found in JonBenet's vaginal tract. Cellulose--as in wood fiber.

Birefringence is a fancy word for 'shiny' and indicates the material is of a crystalline structure (microscopically). That could include either the varnish from the paint brush or talcum. Probably not enough of a sample to discover the source.

Rainsong