Mineral vs Sapphire (stolen from watchuseek)

The yield strength of sapphire is in the neighborhood of 2.25 GPa.
The yield strength of mineral glass is around 75 GPa.

The modulus of elasticity (how easily the material deforms) for the two: sapphire - 250 GPa, and mineral glass - 82.5 GPa.

What does all this mean? It means, for the same diameter and thickness of watch crystal, it takes about 35 times more force to fail a piece of glass than a piece of sapphire, but sapphire will deform about three time more than glass before failing.

Plastics have a yield stength of around 67 GPa, and a modulus of elasticity of 2585 GPa, which means they are about as strong as glass but able to deform about ten times more than glass before failing.

Also, sapphire and mineral glass is tested by dropping a 2.25 ounce steel ball-bearing on a representative crystal from varying heights until the crystal breaks and the total amount of energy needed to break the glass can be calculated. According to ISO 14368, Part 3, mineral glass must absorb 1600 to 2100x10^-4 Newton-Meters, sapphire is only required to 800 to 1800x10^-4 NM.