An unusual metal alloy. The Clockmakers' Guild were supposed to be the only people who knew about it. In any case, it is very expensive. Worth a lot more than its weight in gold. Although unaffected by temperature, the alloy would not have permitted Jeremy Clockson to achieve the complete accuracy Myria LeJean was looking for. So, he built a Glass Clock instead...
Like many readers of Pratchett, I assumed this exotic name denoted a sort of rare and expensive phlebotinum specific to the Discworld, which only an Auditor could reliably obtain (or call into being) in the quantities desired to go into a Glass clock.
well... it exists on Earth and is an alloy of two extremely common metals.
Also known as FeNi36, Invar is an alloy of iron and nickel in the proportion 64:36. It is known as "invar" - invariable - because its thermal coefficient of heat is incredibly low. That is, it hardly expands if heated, a property unique to these two metals in this exact alloy. This makes it a friend to those who are creating extremely accurate scientific instrumentation to a very, very, exacting tolerance. (Science calls this property an "anomaly". In other words, nobody's got a clue as to why this alloy behaves like this). Jeremy Clockson would be extremely excited by its properties and potential.
Whilst invar has an extremely low thermal coefficent, this is not the same thing as saying it has none at all. Thermal expansion, and therefore distortion and degraded measurement, will still set in even at tolerances measured seven or eight zeroes past the decimal point. so a glass clock could not be built of invar alone. (The Wikipedia entry on invar notes that a slightly different formulation has properties analogous to "borosilicate glass" making it useful for use in extreme or hostile environments, such as satellites in space...)
On the date of this entry (08/03/2015) invar cost between $23 - $40 per kilogram. The corresponding cost of gold per kilogram was $37,574.58. But these are Roundworld prices. Either making such a guaranteed precise alloy on the Discworld is technologically difficult, ro else the Dwarfs are chiselling again.