About the Green Crystal in the Milgauss...

[Lol-x]

We look at our watches every day, but we rarely consider the origins of what we are looking through i.e. the sapphire crystal.

The sapphire crystal is an artificial (“synthetic”) man made material that is ‘grown’ over a period of days or weeks.

The growing process is even more complex in the case of the Green Milgauss Crystal.
Corundum is the mineralogical name for the naturally occurring crystalline form of aluminium oxide (sapphire).

The chemical formula for the Synthetic Green Sapphire is Co3+ - Al2O3. In other words synthetic green sapphire is made from Copper and Aluminium Oxide, the latter being the main product of bauxite the principal ore of aluminium.

The production of synthetic sapphire crystals can occur by one of several processes: melt growth, solution growth, or extremely high-temperature, high-pressure growth.

The 4 well known methods of sapphire production are:
1. The Verneuil flame-fusion process
2. The Bridgman-Stockbarge process
3. The Czochralski method
4. The Hydrothermal method

It is not precisely known which method Rolex uses to produce their sapphire crystals, but rumour has it that Rolex uses the Hydrothermal method of producing the green sapphire crystals.

The Hydrothermal method is based on the transformation of melting crashed sapphires and deposit of solution around the beryl seed in a hydrothermal environment. The crashed low grade sapphires, known as nutrient, are melted along with the color inducing chemicals (copper is used for obtaining a green tint). The seed, being a larger and more solid element in the process, attracts smaller particles of the same origin and the growth process begins, depositing layer upon layer on itself. This is the reason for undulated growth lines in hydrothermally grown gems. The process takes around four weeks. If the process is slowed down, the “layers” are deposited at a slower rate, resulting in growth lines being less defined and more difficult to detect.

The process starts by gradually heating the autoclave. Subsequent rise in temperature leads to rise in pressure, as a result of liquid expansion inside the inner vessel. Once the temperature rises to about 600°C and pressure is around 800 PSI, the process reaches an ultimate equilibrium and a melt process begins. Smaller particles are attracted by the larger element, the seed, hence starting the growth process.

http://tairus.com/index_files/emerald.htm

The hydrothermal synthetic sapphire 'growth' process is similar to Synthetic Emerald production process.


Synthetic sapphire ranks a 9 on the Mohs Scale, this means that one needs to avoid exposure to diamond jewellery, and avoid striking ones watches against artificial stone and simulated stone surfaces. Such surfaces can often contain materials including silicon carbide, which, like diamond, are harder than sapphire and thus capable of causing scratches.

The slow run out of the green crystal milgauss (and the higher cost than the clear crystal models) may well be explained by the complex process involved in the growth of the green crystals. The addition of copper to the process to obtain precisely the right amount of green tint, and the precise pressures and temperatures to yield a perfect product are liable to failures occurring in the ‘growth’ process.