There are usually three types of glass used in watchmaking to protect the dial: synthetic, mineral and synthetic sapphire. The former is generally adopted on low-end watches: it is particularly elastic and thus able to withstand even shocks of a certain importance; it has a tendency, however, to scratch and it is not always easy to polish it to restore its transparency. Mineral glass belongs to the category of plain glass used in the past and is subjected to tempering processes which increase its resistance to shocks and scratches.
Synthetic sapphire, the newest and also the most expensive, deserves a separate mention. Extremely hard, it requires long and delicate processing. Practically unscratchable, it manages to retain its shiny appearance for a very long time; it is also rather resistant to shocks (especially if the thickness is consistent) but, due to its particular physical characteristics, it shatters into tiny fragments in the rare cases in which it might break (unlike synthetic glass, which never breaks into tiny parts, thus making it easier to replace). This is why it is necessary to take the watch in the event of a sapphire crystal breakage immediately to have it repaired and undergo a complete overhaul as the fragments could also damage the movement.
Both sapphire and mineral glass can be treated with anti-reflective coating for enhanced readability. If carried out both externally and internally, the surface almost seems to disappear from view, which is why the glass is particularly valuable.
