Melting point of obsidian
Obsidian

This is a volcanic glass, formed when rhyolitic lavas cool too quickly for crystals to form.  From a chemistry point of view, it's a mixture of silicon dioxide, aluminium oxide, with sodium ,potassium, calcium and iron oxides in various quantities. These lavas will also contain significant volatiles (water and $\ce{CO2}$) held in solution by pressure, although they should be lost at the pressures required for glass formation (higher pressures means slower cooling due to more insulation).

Melting

Note that when molten, rhyolite lavas can have a very high viscosity. So it's possible that you have melted your sample, but just didn't give it enough time to flow. Indeed, with substances like this you will see partial melting - some components will melt at a lower temperature than others. If you are getting bubbles, this would indicate that your glass formed under enough pressure to retain some $\ce{CO2}$ and/or $\ce{H2O}$ in its lattice, and you are now allowing this to be released by heating at surface pressure.  This is all a bit speculative, without a closer inspection.