Why does quartz shatter and break when exposed to heat?

A single crystal of pure, clean quartz actually tends to handle heating very well. However, milky quartz and quartz composed of many crystals can shatter when heated. This is because quartz is a poor conductor of heat, so the material heats unevenly (unlike metals, which dissipate heat well internally). Uneven heating results in uneven expansion, and because of the rigid bonding in quartz, this can only be relieved by breaking bonds and cracking the crystal. Milky quartz is also thought to contain impurities of water and gases, which expand even more greatly when heated, putting pressure on the inside of the quartz.