Could Jarn really have made glass?
Sure, if his ability to heat things up with his mind was sufficient to melt the crystals in sand or rock completely. Volcanoes and lightning strikes do it all the time, producing obsidian and fulgurites, both glasses. But he wouldn’t make modern window glass by accident!
Scientifically a glass is an amorphous (non-crystalline) solid. Most are made by melting or dissolving something which may be crystalline to start with and then cooling it so fast that crystals have no chance to form. This can be done, for instance, with sugar. In fact, sugar glass was widely used for breakaway windows in Hollywood special effects. It had to be made up on the spot, as it would absorb water from the air, but if you’ve seen a stuntman thrown through a window, the chances are that window was made of sugar.
When sand is the raw material, silicon dioxide (quartz) is usually the main ingredient. Pure or nearly pure silica sand would give silica glass, which is used when expansion or contraction with temperature would be a problem, or at high temperatures. But it is hard to make, so most modern glass has two “impurities”: soda (Na2O) and lime (CaO). Lots of other things may be added in smaller amounts as well. If Jarn found a sand with these minerals as impurities, he could indeed have produced a crude form of what we would recognize as glass, and given his mental abilities he could have formed it into transparent sheets.
Why these particular impurities? Pure silica sand melts at a very high temperature. Mixing it with soda reduces that temperature—but the resulting glass, like sugar glass (though not quite to the same extent!) is water-soluble. Adding lime and a couple of other trace ingredients greatly reduces the solubility and produces greater chemical stability.
Transparency is a property of many large crystals, such as quartz, Most rocks made of silicon dioxide, such as flint or jasper, are not transparent simply because they are made up of many tiny crystals, and light reflects off the crystal boundaries. Glasses, having no crystal boundaries, are often transparent.
Lack of transparency in a glass may be due to bubbles or to the inclusion of elements which color the glass. Many sand grains, for instance, are yellow because of a coating of iron oxide. This would color glass made from yellowish sand, though the color is more green than yellow. Small amounts of various chemicals are in fact used to color glass deliberately.
So Jarn could have made glass by accident and then learned what he had done and how to refine the process from his computer. His fusing of dirt would have been more akin to firing ceramics. But these were not arts he could have taught to anyone else.