Sapphire Crystal:
Why It's Essential

The crystal of your watch is the first thing you look at and the first thing that takes hits daily. Choosing the right material means choosing how long your watch will look as good as new.

The Three Types of Crystal in Watchmaking

There are three main materials used for watch crystals: mineral glass, acrylic glass, and sapphire crystal. Each has its characteristics, advantages, and disadvantages. Here's what you need to know.

Material	Hardness (Mohs)	Scratch Resistance	Price	Used by
Acrylic (plexiglass)	3	Very low, scratches easily	Very low	Entry-level watches
Mineral glass	5 to 6	Correct, resists minor scratches	Low to medium	Mid-range watches
Sapphire crystal	9	Excellent, virtually unscratchable	High	Our Seiko mods

Synthetic Sapphire: What is it?

The sapphire crystal used in watchmaking is not natural sapphire extracted from a mine. It is synthetic corundum (Al₂O₃ — aluminum oxide) produced in a laboratory by a very high-temperature fusion process. The result is chemically identical to natural sapphire, with the same hardness properties.

On the Mohs scale, which measures the hardness of minerals from 1 (talc) to 10 (diamond), sapphire reaches 9. Only diamond is harder. In practice, this means that only materials containing diamond or corundum particles can scratch a sapphire crystal. Beach sand, keys, coins, concrete — none of these elements will leave a visible mark.

Anti-reflective sapphire crystal — transparent, solid, and protected from daily scratches.

The Anti-Reflective Treatment

Raw sapphire crystal has a flaw: it reflects light more than mineral glass. Without treatment, the image in the crystal can be distracting in bright light. This is why the vast majority of sapphire crystals used in quality watchmaking receive an anti-reflective coating.

This treatment consists of depositing one or more layers of low refractive index materials on the surfaces of the crystal (internal, external, or both). The result is a crystal that allows up to 99% of light to pass through, without glare, with optimal dial readability regardless of the light conditions.

Mineral Glass vs. Sapphire: The Everyday Difference

Mineral glass is the standard crystal used on production Seiko watches and most mid-range timepieces. It is decent but scratches over time, especially when in contact with abrasive surfaces like concrete, roughcast, or sand. These micro-scratches accumulate and eventually dull the appearance of the dial.

Sapphire crystal resists all of this. After 5 years of daily wear, a sapphire crystal remains as transparent and smooth as the first day, provided it is not directly hit against a hard corner, as sapphire, although very hard, can crack under a violent impact (its hardness is high but its shock resistance is lower than that of mineral glass).

What Sapphire Crystal Does Very Well

• Resists everyday scratches
• Remains perfectly transparent for years
• Offers optimal readability with anti-reflective treatment
• Maintains the watch's premium appearance over time

What You Need to Know

• It can crack under a violent, direct impact on a hard corner
• It is more difficult to polish than mineral glass if damaged
• It is heavier than acrylic glass (imperceptible difference when worn)

Why We Use Sapphire on All Our Watches

At Modding Watches, anti-reflective sapphire crystal is a standard across all our creations. This is not a marketing argument; it's a fundamental choice that reflects our vision of what a quality watch should be.

We build watches to last. A sapphire crystal that remains impeccable after years of daily wear is the guarantee that your watch will be as beautiful in ten years as it is today. This is consistent with our artisanal approach: every component is chosen for its long-term quality, not to cut costs.