Quartz crystals are piezoelectric, meaning they convert mechanical energy, such as vibration, into electrical energy, but they do not convert heat energy to AC electrical energy. For a quartz crystal to generate voltage, it must be deformed along a specific axis, which is not achievable when subjected to random impacts from gas molecules. Immersing a quartz crystal in a hot gas would not produce any useful vibration or deformation, thus failing to generate electrical energy. The discussion emphasizes the importance of understanding the specific conditions required for piezoelectric effects. Overall, quartz crystals cannot directly convert heat energy into electrical energy.