Tornado Alley (PROG)

[DesertDrifter]

parabolic mirrors were first described by the chinese:

Among the first known tools made to harness the power of the Sun are Yangsui burning mirrors, made in Ancient China, around 3,000 years ago. Like a magnifying lens these concave bronze mirrors reflected and focused the Sun's rays to a specific point. On a bright sunny day this produced enough heat to start a fire.

Archimedes is said to have created burning mirrors to set enemy ships on fire, but there is no evidence and many consider it a myth.

As far as melting granite... not a simple thing to do at scales needed for construction, etc. with their tool set. But I would imagine small molds could be made of wood or soapstone and then have partially cooled lava that is still plastic moved into the mold. people still handle molten lava in buckets sometimes. If lava cools slowly, it forms crystals, but if it is doused with water it solidifies without crystalline structure, and forms rocks such as obsidian or dacite that are devoid of crystalline structure.