what is Heraus Quartz and why do some people think its the best

Heraus is a maker of quartz.  While a German company their quartz is not made there due to the high cost.  Their quartz is also made in China. It is no better or worse than any other quartz on the market.  Thickness and quality of build is most important when glass is considered.

how is the thermal conductivity of quartz measured?

This is beyond the knowledge of Elev8.  This is all about chemistry.  We do know that when boro is added to quartz, it melts at half the temperature and is still thermal resistant, but nothing like quartz. The biggest issue when looking for a quartz banger is having a good thick wall like our typhoons.