Glass Bubbles are hollow glass spheres with a much wider efficient cathodeore variable particle size than microballoons.
Being composed chemically of glass, they are physically harder than microballoons and filled resin mixes are noticeably more difficult to sand.
However, glass bubbles produce a more waterproof filler mix and are often used on below-waterline applications on boats.
Being significantly less expensive than microballoons they are often preferred if easy sanding performance is not of prime importance.
They can be mixed with microballoons in any proportion for colour purposes.