The glass or often crystal protects the interior of the watch, thus dial, hands and movement, against damage through dirt or moisture. In general, watch glasses can be classified in three groups: plastic glasses or acryl, mineral or sapphire glasses. They mainly differ in their hardness and stability.
Plastic glasses largely disappeared
Acryl glasses, widespread with watches in the 50’s, are nowadays hardly used anymore as they are in comparison to mineral crystals 20 times softer and are thus more sensitive to scratches. Sapphire glasses are the most scratch-resistant glasses there are, whereas the name can be misleading as they are no glass. They are a synthetic product which is produced with a Verneuil process. The name rather refers to their hardness, which is comparable to a sapphire.
Mineral glasses are more stable, sapphire has higher scratch-resistance
By contrast, mineral glass is a crystal glass that resembles in its structure quartz. Being extra hardened through special production processes (e.g. heating or vaporization), almost all modern mineral crystals have an even more resistant surface than a few years ago. The glasses thus achieve a hardness of around 900 Vickers. Even though sapphire glasses achieve around twice as many Vickers, they thereby also become more brittle. That means, if a watch with a sapphire glass falls to the ground, the likelihood that the glass bursts is higher compared to one with a hardened mineral crystal, as sapphire glass is more sensitive to shock.
Domed glasses are much more expensive
The material cost for a sapphire glass is today only insignificantly higher as for a high quality, hardened mineral crystal. That however only holds for flat or slightly domed watch crystals. Stronger domed sapphire glasses on the contrary are only applied to luxury watches with according sales prices, as the production process is still relatively new and expensive. A double-domed mineral crystal is likewise already more expensive in production then a normal, flat sapphire glass.
Mineral crystals are sufficient for common use
To sum up, extra hardened mineral crystals, like the double domed glasses of the Circula watches, are absolutely sufficient for the common use and should get non or only minimal scratches throughout their lives. Sapphire has certainly a higher scratch-resistance, but also bursts more easily.