Rolex and the finest watchmakers have turned to sapphire crystal to be the face to their exquisite timepieces.  There are several reasons for this.  However, to really understand the decision to use sapphire it is worthwhile to look at the various alternatives.  Not all watches manufactured today utilize sapphire.  Besides sapphire, acrylic/plexiglass/hesalite, and mineral/hardlex are often used.

Until 1970, Rolex utilized acrylic as the glass for their timepieces.  Acrylic is a low cost plastic and is quite hard.  It is practically shatterproof.  It is so hard that when it gets scratched, people often use a compound such as polywatch to buff out scratches fairly easily - returning the beauty of the glass to its original form with a crystal clear polished look.  Even today many timepieces utilize the acrylic crystal because of the low cost and ease to produce the glass.  However, the acrylic glass tends to be thicker than sapphire crystal glass, which can add weight and thickness to the timepiece itself.

Companies like Seiko have opted to upgrade their glass to a mineral based glass which they call Hardlex.  This is more costly than acrylic, but less than the sapphire.  It is shatter resistant and is less scratch prone than acrylic.  It is also fairly easy to manufacture and shape.  Mineral based crystal is one of the most common glass types used on timepieces - especially in the under $300 watch market. Like the acrylic, the mineral based glass can be buffed and polished to remove scratches.

Sapphire crystal requires a flame fusion process which requires diamond saws to grind and shape the glass, so it drives the actual cost of the glass to over $100 in the aftermarket.  Because of this, you will not likely see any sapphire timepieces under $300.

Rolex clearly wanted to create a scratchproof substance, which is both hard and durable.  It developed a synthetic sapphire crystal which is so hard that only diamond registers as a harder substance.  The hardness keeps the crystal from getting scratched because only a substance like a diamond really has the ability to create a scratch across a sapphire surface.  Although sapphire is far more scratch resistant than acrylic it is possible to shatter.  A shattered sapphire crystal could damage the Rolex timepiece significantly.  However, the rarity of a shattered crystal is what Rolex was banking on.  They found that it was far more common for an owner to scratch their acrylic glass than it was for them to shatter the sapphire.  A sapphire crystal would basically look new forever.  And, Rolex is all about timeless beauty.

Another key reason Rolex liked sapphire crystal is for the smoothness of the surface.  Because of the lack of pores in the sapphire surface, Rolex is able to add an antireflective and antiglare coating to the bottom side of the sapphire crystal.  This adds clarity to the timepiece even in strong sunlight.  It wouldn't be possible to add this antireflective coating to mineral or acrylic.  This antireflective coating gives a bluish glow under certain light, but keeps the timepiece from appearing cloudy.

Sapphire also has interesting refractive qualities.  For example, if you compare domed sapphire to domed mineral, you would see a distortion of the watch-face by the mineral crystal at certain angles.  However, the sapphire will produce a clearly visible timepiece face at nearly any angle. Although this is hardly noticeable unless you turn your timepiece to the side and look through the crystal at a sharp angle, Rolex does care about the details.

One last feature of sapphire crystal is its ability to keep water from streaking across the surface.  If you place a drop of water on a sapphire and a mineral surface, you would see a significant difference.  On the sapphire, the water will bead up and keep shape.  On the mineral it will spread out.  If you turn the crystals vertical, the sapphire water bead will gracefully and completely fall off the face of the sapphire keeping a very clean and clear appearance.  The mineral would streak and the water will leave a trail as it falls off the crystal.  The benefits for Rolex here are obvious.  Many of Rolex timepieces are engineered for diving and swimming.  The Submariner is a perfect example of this.  A sapphire crystal would help retain visibility and repel water naturally as swimmers exit the water.

Rolex started rolling out sapphire crystal on their timepieces as early as 1970 on their Oyster Quartz, and then Submariner in 1981.  By the mid 1990's Rolex had updated most of their timepieces to all utilize sapphire crystal, and today all Rolex timepieces have sapphire crystal.

Rolex has added the cyclops lens to the top of the sapphire crystal to provide access to the date window in models that contain a date window.  If you turn the crystal with the cyclops lens to the side, you will see that the lens is added to the sapphire glass rather than being molded inside the glass itself.