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Sapphire crystal or mineral glass?

Of course it is a shame if the surface of watch glass gets scratched. Other people might not even notice it, but if you are aware of it, it is not easy to overlook. Here at Rohje, we've aimed to make a watch that won’t get scratched at all. That's possible when focusing on details.

Most commonly the glass of watches is made out of mineral glass or sapphire crystal. Sometimes even acrylic, meaning plastic.

We found that sapphire crystal, also called as sapphire glass is the choice that serves Rohje watches the best. Next I’ll explain why.

Sapphire glass is made synthetically and isn’t actually considered as a glass, but crystal. Sapphire crystal is made in very high temperatures, and it is constructed of crystalline aluminium oxide. It is then cut with a diamond saw, grinded and polished to the finished “glass” product. Sapphire crystal can sustain scratches very well and can only be scratched with certain rare materials, basically diamonds and especially hard rock types.

Mineral glass, a common material for watch glasses a swell, scratches more easily but can withstand impacts better. Similarly, acrylic glass, meaning a “glass” made of plastic, can take hits because of its flexibility but scratches very easily.

We saw that with standing scratches and with standing impacts are opposite qualities. The harder and less likely to be scratched the glass is, the less it can take impacts. Hardness is measured with the Mohs scale, which measures mineral hardness on a scale of 1 to 10.

Diamond is the hardest mineral, with a value of 10, and can scratch all other minerals. Mineral glass is around 5-6 and acrylic around 3. The Rohje Sapphire crystal is 8-9 on the Mohs scale, and therefore nearly unscratchable.

We have focused on the details of our chosen sapphire crystals. The reflection of the sapphire glass can be diminished by adding the smallest anti-reflective coating. That is why you can see a tiny amount of blue on the face our Rohje watches when looking at them from a specific angle.

The glass of e.g. Rohje Adventurister watch is coated with this anti-reflective coating. The glass could be treated on both sides, but the outside coating could wear off and make the glass splotchy. We wanted our watches to work well but also look good for a long time, which is why we ended up coating only the inside of the glass.

Take a look at our watches here