Everything you Need to Know about Watch Crystals

If you’re an owner of one watch or several, you probably spend a good amount of time looking at your wrist. Maybe, you just take a quick time check or a lingering glance to appreciate the fine workmanship. The sheen of the dial might catch your eye, or you might touch it to rotate the  bezel or wind the crown. Either way, you need to be able to see the dial clearly in all lighting situations to be able to ascertain the time, date, or day quickly. In order to do this, the glass that covers your dial needs to be clear, transparent and reflection-less. The use of sapphire crystal makes this possible. 

What Is Sapphire Crystal? 

Sapphire watch crystals are called crystals because they are manufactured using the same technique used to make industrially-created gemstones. Sapphire crystals are called boules, referring to their cylindrical crystal shape. Sapphire crystal retains the physical strength of a gem due to its latticed crystal structure. It rates a 9 on the Mohs scale, making it highly resistant to nicks and scratches. It also contains the clarity and brilliance you see in gems, making it the perfect protective material for high-end timepieces or smartphones. The gems we know as sapphire and ruby are both names for corundum, the same material that is used for the synthetic rubies in watch movements. So, corundum is used both for the external beauty and strength of the watch (as sapphire glass) as well as the accuracy of its interior movement (as synthetic rubies).  

What is Hesalite? 

Perhaps most famously known for its use in the Omega Speedmaster Professional Moonwatch, Hesalite is a hard acrylic used in watches before many watchmakers transitioned to using sapphire crystal. This tough material was highly scratch-resistant. However, it was more prone to fogging or losing clarity over time due to UV light exposure, humidity or temperature changes. According to legend, NASA requested a Hesalite crystal for the watches used by astronauts. If the watch shattered in orbit, it wouldn’t send minuscule glass shards floating through zero gravity. (However, this story is probably untrue or exaggerated.) Up until recently, Omega still sold a version of its Moonwatch with the original Hesalite crystal, which some collectors preferred for the historical accuracy of the reproduction.  

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What is an Anti-Reflective Coating? 

Also called AR Coating or ARC, this thin coat is necessary for eliminating glare and making the watch crystal seem to disappear when you glance at it. As important as the watch crystal is for watertightness and strength, it also needs to appear nearly invisible to the wearer. That’s a tough task. Anti-reflective coating on either or both sides of the crystal can help it seem insubstantial and weightless, even though it’s one of the hardest and strongest materials used in watch manufacturing. Some collectors prefer ARC on only the underside of the crystal. If applied to the top of a watch crystal, hairline scratches or nicks on the ARC can interfere with the clarity of a crystal, even though it prevents glare and flashing. 

Are There Any Unusual or Unique Watch Crystals? 

Rolex uses a green crystal in the manufacture of the Milgauss. The thin green line of the crystal circling the dial is a hallmark of this reference. Although Rolex has patented many of its materials and processes, they noticeably don’t have a patent on this green crystal. In fact, Rolex claims that the process of manufacturing this crystal is so complex that they don’t envision any other company attempting it. Experts surmise that the green tint is due to the addition of copper or aluminum oxide during the crystal formation process. Of course, it’s difficult to imagine another brand using a green crystal without feeling imitative or derivative. The green crystal on the Milgauss contrasts well with the striking orange second hand, and plays nicely with a blue, gray, or black dial. Using a rubber strap on a Milgauss can highlight the use of this unique crystal color. Try it with an orange strap to match the second hand or a green rubber strap to highlight the Milgauss watch crystal. Consider it an homage: a subtle but noticeable nod to an under-appreciated watch component.    

Written by Meghan Clark

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