question regarding jewels

[nmanousos]

if cost were not a concern, would it be feasible to use sapphire (or even diamond) for jewels in a movement? if so, would lubrication still be needed?

thanks,
Nicholas Manousos

[whiskeybelly]

Short answer: yes, and yes, lubrication would still be needed.

Long answer:
Sapphire and ruby are essentially one and the same, being composed of crystalline aluminum oxide (corundum), the difference is the color- hardness is the same. Watches have been produced with various colored sapphire/ruby, from clear to light pink to blue to deep red. The various colors are a result of impurities in the corundum, chromium in the case of red.

Diamond has also been used, notably in marine chronometer balance jewels (in particular end stones) where the balance is tremendously massive in comparison to the size of its pivots. I've seen a number of marine chronometers where the steel balance pivot has worn a divot in the diamond end stone, with the pivot showing normal or no wear. This has been explained to me as happening as a result of steel's affinity for carbon, which diamond is composed of.

I know that diamond abrasives can not be used on steel when the abrasives (a diamond wheel for example) are run at high surface speeds as the wheel will be utterly ruined. However, diamond bench stones and files are commonly used on steel with no ill effects.

The surface speed of the pivot end rubbing on the diamond is quite slow. The pressure at the pivot tip is quite high though- and perhaps this is why end stones are seen worn to a certain point (the pivot is radiused on the end, so as it wears the stone contact area is increased and the pressure distributed from almost a single point to a comparatively much larger area). This may explain why diamond endstones are encountered worn to approximately the pivot diameter but the pivot never actually 'drills' a hole through.

Ruby endstones also show wear in come cases when used in pocket and wrist watches. Another explanation for the wear of both ruby and diamond stones by lowly hardened steel is the improper use of abrasives when finishing pivots. Abrasives can become imbedded in the steel, creating a lap which over time eats away at the stone. Generally pivots are finished by a burnishing operation rather than with abrasives, which avoids this as well as increasing the surface hardness and wear properties of the steel.

From a practical standpoint, especially in wrist and pocket watches, diamond offers no real advantage over sapphire/ruby.

[nmanousos]

thank you very much - very thorough explanation!