Crystal

Two standards series cover the part, and both name only two of its three materials. ISO 14368-3:2003, Mineral and sapphire watch-glasses — Part 3: Qualitative criteria and test methods, edition 1, published August 2003, "specifies the nomenclature of defects and the test methods to check them, with minimum physical and aesthetic requirements, for circular mineral and sapphire watch-glasses". The Swiss horological standards list published by FH carries a parallel four-part series, NIHS 61-11 to 61-14, Verres de montres minéraux et en saphir, covering dimensions and tolerances, fixing to the case by adhesive or gasket, qualitative criteria, and anti-reflective treatment. Acrylic appears in neither series.

Sapphire in a watch is synthetic corundum: aluminium oxide, written Al2O3. The Handbook of Mineralogy (a mineralogical compilation issued by Mineral Data Publishing, not a standards body) gives corundum's formula as "Al2O3", its hardness as "Hardness = 9" on the Mohs scale, and its measured density as "D(meas.) = 3.98–4.10". Nine is the value for the mineral species, on a comparative scratch-resistance scale; it is not a measure of toughness or of resistance to a blow.

What this Codex does not do: give Mohs figures for mineral glass or for acrylic. No mineralogical or standards authority publishing values for those two materials was located and read, so the numbers that circulate in the trade are not repeated here. Only the titles and published scopes of the ISO and NIHS texts have been read; their full texts have not.

A note on the word. "Crystal" here is a trade name for the cover over the dial, and has nothing to do with the quartz crystal whose vibration sets the frequency of a quartz movement.