Technical Notes: The Sapphire Crystal

Sapphire has been an increasingly popular material to use for watch crystals over the last ten or so years. Due to its impressive hardness qualities, sapphire provides incredible scratch resistance when compared to acrylic or hardened mineral. However, as with many materials rating high on the Mohs scale, hardness often has an inverse relationship with brittleness – making sapphire more prone to shattering. This is where acrylic has the advantage. Despite the scratch resistance and overall appearance of quality, many watch lovers still favor acrylic over sapphire for its ease of polishing out scratches and the unlikelihood of shattering, In fact, it takes little more than some household items to buff out scratches on acrylic. Hey, if it’s good enough for walking on the moon, it’s good enough for the average Joe.

Although the popularity has been more recent, sapphire has been utilized here and there since about 1930. Jaeger LeCoultre was said to have been the first watchmaker to use a sapphire crystal, with Omega being the next, some 30 years later. At the time, sapphire was difficult to work with, as well as cost prohibitive. It wasn’t until Rolex began using sapphire crystals on most of their watches in the mid-1980s that the new crystal material was really put on the map.

You should all be somewhat familiar with natural sapphire, but the sapphire used for watch crystals is synthetic. There are a number of ways sapphire can be synthesized, or grown: through melting, vapors, solutions, gels, or under high pressure. We’re going to focus on melting, since this is the most common practice within the industry.

In general, the idea of growing sapphire crystal is to place a sapphire seed crystal in a crucible filled with molten aluminum oxide (Al₂O₃). A seed holder keeps the crystal and melt segregated throughout the process, which gives the seed empty space to grow, often upward into a cylindrical shape. The seed holder inside the melt is simultaneously cooled, creating the proper environment for the crystal to grow. Fine-tuning the temperature during growth allows the technician to control the size and shape of the crystal. It’s at this point where the method of extraction determines the specific method used.

The Czokhralski process, originally created in 1918 and first used with sapphire in 1960, involves literally pulling the sapphire crystal out of the crucible. What you get isn’t unlike sapphire on a stick – well, it’s a bit more scientific than that, of course, but you get the point. One of the key benefits of this method is that during growth, the technician turns the seed to precisely control shape and size. Still widely used today, the Czokhralski process enables remarkably high quality sapphires.

In 1964, another extraction process was created, which is known as the gradient-freeze technique. Instead of pulling the crystal out of the melt, it’s completely cooled horizontally inside the crucible. This method has evolved a bit, but is used today with great effectiveness.

Sapphire crystal growth through gradient-freeze

Taking parts of the gradient-freeze technique and the Czokhralski process, the Kyropoulos method is another extensively used crystal-growing technique. Throughout the process, the seed is turned just as it would be with the Czokhralski’s method. Then, in lieu of pulling the crystal, the growth is cooled. Between the highly controlled turning and cooling, the Kyropoulos method allows for very large crystals to be produced – an obvious advantage for mass production.

Don’t worry; the fun doesn’t end there. All we’re left with now is raw sapphire crystals that need some shaping and polishing. If you could just picture the clay-molding scene from Ghost, it’s nothing like that. As you know, sapphire is hard as hell, which makes shaping and polishing quite difficult. The only things that can cut, grind and polish sapphire are other pieces of sapphire, or diamond. Depending on the desired shape, cutting and grinding phases range from tedious to über tedious. It may go without saying, but flat sapphire crystals open up on the tedious end and the domed crystals are on the über end.

Polishing can be, and is done by machine. But in some cases, especially with the more complex crystal shapes, polishing requires some human help. As someone who’s attempted to polish a sapphire crystal, it takes copious amounts of elbow grease. The other tricky part is creating an even polish. The slightest differences in thickness can distort the view of the watch dial – and nobody wants that.

Because sapphire is brittle, at any point during the shaping process, minute cracks and breaks can show up. Although they may not be visible to the naked eye, they’re unacceptable for use by any watchmaker claiming to provide quality materials. Post-production inspection can be time consuming, and revelation of flaws in the sapphire can be very costly. The entire process is highly intricate, and rife with complex steps that can easily ruin a batch of sapphire.

Even though the growth of any synthetic sapphire is at least relatively difficult when compared to acrylic or hardened mineral, there are absolutely differences in quality. Flat sapphire crystals are more likely to shatter under blunt force trauma versus domed crystals. Thickness also plays a role. Despite sapphire’s brittle tendencies, it’s not an easy task to shatter a thick crystal. Thinner crystals, used by watchmakers looking to cut some corners but still give the appearance of quality, are the ones you should be careful with when you come down with a case of the butterfingers.

The sapphire crystal trend has come on strong, as it seems every watchmaker and their mother is using them. With such widespread use, it no longer represents a quality watch in and of itself – which can largely be credited to the advancements in crystal growing machinery and techniques. Lovers of acrylic need not worry (hello, vintage), but others would do well to stay focused on the sum of the watch parts, not just buzzwords like ‘sapphire crystals’. Nevertheless, manufacturing sapphire crystals is a unique and intricate process. It will be interesting to see how it progresses, especially if other industries (like mobile phones) continue sapphire adoption.

Are you a sapphire or acrylic watch person? Tell us in the comments!

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