That one I linked was rated 12.8W/mK (but amazon listing, so take with
pinch of salt), others from more reputable sources vary from 1.5 to
around 10 for cheaper silicone/ceramic ones and as high as 35W/mK for
expensive silicone/graphite composite pads (which are electrically
conductive). Some super-specialized oriented-grain graphite/graphene
composite ones are available with thermal conductivies exceeding that of
solid copper but are £££££ and only available in quite thin (<0.1mm) sheet
Glass is normally considered to have a conductivity around 1W/mK and
porcelain around 1.5W/mK
Steve