The most popular material for watch cases, bracelets, buckles and clasps is stainless steel and in particular a steel alloy called 316L. 316L is regular steel with 16% chromium added (the 16 part of the number), 10 to 14% nickel, and with a reduced carbon content (the L bit) of not more than 0.03%.
The addition of chromium and nickel means that 316L steels present very good corrosion (rusting) resistance as well as being nice and shiny. They’re not completely rust proof though, so you should wash down a stainless steel divers watch with fresh water after use. There’s a clue in the name, ‘stain-less’, not ‘stain-free’, which most architects don’t appear to have noticed and has led to the tendency for buildings that use it as an outside material to take on a horrible stained appearance after a few years.
The low carbon content in 316L reduces the alloys brittleness and increases its toughness. With watch cases it's not just ultimate strength that's important but also the toughness and ability to absorb impact energy. Glass for example has an ultimate yield strength greater than most steels, but because of its low ductility and low toughness it's too brittle to make a very good watch case material. In case you were wondering about ceramics, they're pretty similar to glass in material behaviour.
316L is often called surgical or marine grade steel as it is useful for these applications. You can also refer to it as 'knife & fork' grade steel as most cutlery is 316L. A few watch makes use 904L stainless steel, which has a lower chromium content and therefore is slightly duller in appearance.
Some parts of OFFSHORE Professional chronographs use 316H stainless steel, which has a higher carbon content to increase yield strength. Notably the double ended screw pins in the metal bracelet are 316H to ensure they can be undone and done up several times without stripping their threads.
The nickel content of alloy 316L watch cases can cause an allergic reaction on some peoples skin, although this is very unlikely on an OFFSHORE Professional as the back part of the case that rests on your wrist is sapphire. If you have a nickel reaction to stainless steel then you need a hypo-allergic watch material, which essentially means you need to buy a Titanium or Gold watch. If your wife/girlfriend comes up with this as a reason for needing a gold Cartier then full credit to her.
Titanium watches are typically made from standard aero-space grade titanium alloy, which consists of 90% titanium, 6% aluminium and 4% vanadium. ‘Alloy’ just means a mix of different metals, so a stainless steel watch is also an alloy watch, and an alloy wheel on your car could be made from pig iron and tin and still be an ‘alloy’.
Titanium is more expensive than stainless steel, but given the small amount of material actually used the cost difference is insignificant. Titanium is almost half the weight of steel so the resulting watch can feel light, which may or may not be preferable. The material is also pretty much impervious to corrosion.
Gold watches come as either 9 or 18 carat, and occasionally 15 or 22 carat. Pure gold is 24 carat, so 9 carat gold is 9/24 ths pure. 9 carat gold is therefore an alloy that's mainly composed of other metals such as silver, copper and nickel. The other metals provide the strength and hardness, but since their is no specific percentages it's impossible to state whether a particular gold watch will be more or less susceptible to dents and scratches than steel or titanium.
Why is pure gold called 24 carat and not just 100% gold? Well, the answer is the same as why there are 24 hours in a day; it’s all due to the Sumerians (the guys that first invented writing in 3500bc) and their use of a base 12 numbering system instead of the base 10 system we use today. As noted on the How a Watch Works page, it's also the reason for 60 minutes in an hour, 60 seconds in a minute and 360 degrees in a circle, as all are base 12 numbering. There is a logic in it though. If each finger represents 12 then it’s quite easy to show a full hour as being a hand held up with all fingers and thumb extended (5 x 12 = 60 minutes). And a full days work of 12 hours can be by 4 fingers and no thumb; each finger now representing 3 hours. A finger has 3 bits to it, bend your finger and you’ll see what I mean. So now you can count off hours on one hand and minutes on the other, and divide things easily into half (6), a quarter (3), a sixth or a twelfth, all of which is impossible to do on your fingers using the modern base 10 numbering system. Incidentally none of this is related to the traditional British two finger salute, which comes from the Hundred Years' War with the French (note 1 at bottom of page) and their high loss rate against the long bow. British archers had the two bow fingers cut off if they were captured, so the salute is a gentle reminder to the French to be careful. I have no idea what the American one finger salute is all about, but I'm sure it has equally significant and historic origins.
Gold watches are often sold as completely hypo-allergic and utterly impervious to corrosion. Whilst this is just about true for 18 carat gold, it’s not the case with 9 carat, which can readily dull and might invoke a nickel reaction to sensitive skin.
A chunky 18 carat watch can be quite weighty since gold is 3 times as heavy as steel. This is not necessarily a bad thing though since a watch is normally worn on the left hand and most men, for no reason ever identified, have stronger right wrists than left ones. A chap with a big gold Rolex is simply someone who must have well developed right wrist muscles, perhaps a writer or something similar, and wants to balance it out a bit.
Sapphire & Ruby
Sapphires feature in the OFFSHORE Professional as the front and back 'glass' and the jewelled bearings of the movement. The sapphire used is an aluminium-oxide synthetic sapphire.
Sapphire has much higher transparency to visible spectrum wavelengths than normal chemical glass, as well as much greater scratch resistance (a mohr's hardness of 9) and a higher refractive index. These mean sapphire glass is easier to see though and less susceptable to reflections or scratches. A thin film vapour deposited flouride coating on the inner surfaces of the sapphire crystals further reduces reflections. The sapphire glass used in the OFFSHORE Professional is grown to a specific crystal orientation before being diamond cut and polished. The crystal orientation and fine polishing increase the overall strength by minimising any surface or internal imperfections and a thermal fit up into the front bezel and rear back-plate radially compresses the sapphire to move any impact loading away from the failure region.
The sapphire used for the jewelled bearings is actually a synthetic ruby, composed of the same aluminium-oxide, but with a small quantity of barium flouride and chromium which produce the ruby red colour whilst maintaining an overall mohr's hardness of 9. Because of their extreme hardness, the jewelled bearings will sustain many years of completely unlubricated use.
Copper features in its pure state or alloyed with other materials in many of the internal parts of an OFFSHORE Professional. Copper is significantly softer than steels and so the inner movement holder, which absorbs a lot of impact energy, is pure copper and profiled to take advantage of this. Similarly, copper is easier to accuralety and finely cut with lazers, and has an ideal density for the weight required to self-wind the mechanism, so features in the self-winding rotor. Copper is also non-magnetic and so is used as a primary alloy component throughout the movement. The movement holder, self-winding rotor and most of the movement is rhodium plated to give the silvery-grey finish visible through the back-plate.
Leathers and Skins
Not a traditional engineering material, but the choice of leather and skin types for the strap, along with its layering and construction, is important to maximise its life and minimise the rate at which it may start to look tired. After a quite extensive testing programme, Teju has been selected as the best material for OFFSHORE Professional straps. Teju is a large lizard (typically 4 feet in length), is not endangered or protected and is native to South American, where use is controlled under managed and monitored programmes. Teju meat is consumed as a delicacy (I tried it when I was in the Amazon and it has a similar texture to chicken and tastes of whatever its been seasoned with) and the leather is an important source of rural and indigenous community income. The leather is softer, more pliable and more durable than cow hide and becomes more supple as time passes which. Teju strength, combined with its durability and patterning, give it ideal properties for watch straps..
Note 1: The Hundred Years War lasted for 116 years.