A crystal is the transparent cover protecting the watch face, made from highly scratch-resistant sapphire, moderately durable mineral glass, or lightweight acrylic.
Sapphire crystal is extremely scratch-resistant, rated 9 on the Mohs hardness scale (diamond is 10). It can only be scratched by materials of equal or greater hardness, such as sapphire, tungsten carbide, or diamond. Regular daily wear rarely causes visible scratches.
Sapphire crystal is synthetic corundum with exceptional scratch resistance and clarity but can shatter under impact. Mineral glass is heat-treated glass that's more affordable and impact-resistant but scratches more easily. Sapphire is preferred for luxury and tool watches.
No, sapphire crystal scratches cannot be polished out with DIY methods. The extreme hardness that prevents scratching also makes it nearly impossible to polish. Deep scratches require professional crystal replacement, while minor scratches are typically left as-is or the entire crystal is replaced.