Strength and hardness
Glass made of artificially produced sapphire crystal (Al2O3 = monocrystalline aluminum oxide). With a hardness of 9 on the Mohs scale it has a considerably higher scratch resistance and a resistance to fracture than mineral glass. With these properties, it is only by exceeded the diamond (Mohs hardness of 10).
There are flat, arched and domed sapphire crystals. The difference in production methods is sometimes considerable and is reflected in correspondingly different costs
- Flat or plane sapphire crystals are relatively easy to produce, since they can be sliced directly from the cylinder.
- Simple curved (domed) sapphire glasses are expensive to produce because they are milled from a block and must be grinded into the rounded form.
- For certain types of watches also highly curved acrylic glass can be suitable. Because this can be easily scratched, there is also an alternative high-domed sapphire crystal. It differs from simple curved by the higher number of radii of curvature and the greater thickness of the blank to be worked. To achieve the different radii of curvature, as a rule, several grinding tools are needed. Also, the polishing causes considerable additional expense.