A 1x1x1 meter ball of glass MIGHT shatter upon first impact with a solid object... if you hadn't let it cool all the way through first. How long would that take? How good an insulator IS glass? I seem to recall the invention of the Fresnel lens as a way to make lighthouse lenses because to make such a large lens in a single piece would require cooling over a matter of years...
Glass is an extremely good electrical insulator(one of the best). I don't know so much thermally.
Fresnel lenses are used, however, because they allow the glass to be thinner, which makes it
1: lighter. A solid glass lighthouse lens made using conventional lens methods would weigh several hundred to several thousand pounds. A fresnel variant is significantly lighter.
2: clearer. Fresnel lenses tend to be between 1/4 and 1/10 the thickness of standard lenses at their thickest point. This means that 75-90% less light is lost to absorbtion, resulting in lower power requirements for brighter lights.
A one-foot diameter glass sphere would be able to smash a human(dwarven) head while sustaining little to no actual damage itself, regardless of how it was cooled.
I recall reading in Popular Science about a way to make glass beads. I think you drop molten glass into freezing water, which results in a drop with a very long tail. The round part can stand up to hammer blows, but cracking the end of the tail causes a cascading energy release, splintering the whole thing.