What are the materials used for watch crystals?

　　What are the common materials for watch crystals? Let us take a look below.

　　The watch crystal is the transparent lens on the face of the watch, also known as watch glass, used to protect the watch face (dial). Based on material, watch crystals can be categorised into three types: plastic, mineral glass, and sapphire crystal. Plastic crystals are relatively impact-resistant, while sapphire crystals are the most scratch-resistant.

　　Acrylic (Plexiglass)

　　Acrylic, commonly known as plexiglass, has a relatively soft texture and high plasticity. Most watches in the past used this material, and it is still used in many watch models today.

　　Advantages

　　The advantage of this material is its excellent elasticity and resistance to shattering, which is why it is also called safety glass. Acrylic has high strength, with tensile and impact resistance 7 to 18 times higher than ordinary glass. There is a type of acrylic that has undergone special heating and stretching treatment; even if a nail is driven through it, no cracks will form. Due to these characteristics, it is widely used in military machinery, such as aircraft cockpit covers, armoured vehicle windscreens, and many early pocket watches.

　　Disadvantages

　　The disadvantages of this material are that it is prone to scratching and has slightly lower transparency.

　　Reasons for selection

　　Although it is prone to scratching, when scratches appear, one can have a watchmaker polish the glass directly to restore the original smooth surface, avoiding the need for replacement. If the scratches are too deep, one can choose to replace the crystal entirely, as acrylic crystals are very inexpensive.

　　Sapphire crystal

　　Sapphire crystal is a synthetic crystal made through artificial compression. Its advantages are high density and resistance to scratches. The hardness of sapphire crystal is second only to diamond, with a Vickers hardness of 2200-2300 (seven times that of steel) and a Mohs hardness of 9. It features excellent hardness, good light transmission, low friction, and high temperature resistance, offering higher wear resistance than mineral or synthetic glass crystals. However, it also has disadvantages: it is not shockproof and is unsuitable for improper handling such as knocking, heavy pressure, or dropping. If your watch falls to the ground or is struck, the sapphire crystal is highly prone to cracking.

　　Sapphire crystal watch glasses are generally used for high-end watches.

　　Maintenance

　　Sapphire crystals do not require much maintenance; dust and fingerprints can be wiped off with a soft cloth. However, be mindful of oily stains.

　　How to identify a sapphire crystal

　　For legitimate brand watches, sapphire glass is usually printed on the case back, indicated by the English words SAPPHIRE or Sapphire Crystal. Nowadays, even on replica or counterfeit watches, the use of sapphire glass is quite common. To test the hardness of the watch glass, you can use a sharp metal object to scratch the edge of the glass in the least conspicuous area. If it feels very hard, as if the metal is sliding over it, and the glass is not scratched, then it is likely sapphire glass.