Is sapphire crystal unbreakable? The video guide

Chapters

1. 0:00Scratch-proof is not unbreakable
2. 0:12Synthetic corundum
3. 0:339 on the Mohs scale
4. 0:49Why it hardly scratches
5. 1:07Hard but brittle
6. 1:16The steel ball test
7. 1:31Mineral glass and acrylic
8. 1:46Its real enemy: a knock

Transcript

A sapphire crystal is scratch-proof, or nearly. Unbreakable, no. And those are two very different things. This sapphire doesn't come from a mine. It is synthetic corundum, aluminium oxide. A very pure powder is melted in a flame hotter than two thousand degrees, and it crystallises. The process dates from nineteen hundred and two, and it is still in use. On the Mohs scale, corundum sits at nine, just below diamond, at ten. Measured under the ISO standard for watch glasses, it is about three times harder than mineral glass. That is why it hardly ever scratches. Sand and dust contain quartz, harder than mineral glass, but much softer than sapphire. To scratch it, you need something at least as hard: diamond, or certain abrasives. But hard does not mean tough. Sapphire is brittle: it does not bend, it snaps. The same standard checks this with a steel ball, dropped from higher and higher. At the same thickness, sapphire breaks in the same range as a good mineral glass, sometimes even sooner. Mineral glass, being softer, scratches more quickly. Acrylic, often called Plexiglas, scratches more easily still, but a light scratch comes out with polishing paste. In everyday wear, sapphire is the safest: scratches come from grains harder than the glass, and almost none are harder than sapphire. Its real enemy is a sharp knock, against the corner of a table or on a tiled floor. Discover a whole selection of watches at Montres Outlet.