Said of a mineral crystal or fragment that will not melt in the hottest flame produced by a hand-held blowpipe or blowtorch, i.e., around 1,500 degrees C. The bronzite variety of pyroxene, e.g., has a melt point of approx. 1,400 degrees C and is said to be practically infusible; quartz, with a melt point of about 1,710 degrees C, is infusible.