700 °C insulating material used for a spacer
If you need to machine it, Macor is a possibility 1.46 W/m-K. Good to about 1000°C.
There are also some types of mica-silicone that are barely within that range for intermittent use, but not for continuous exposure.
You can get commercial tubular spacers in Steatite good to 1200°C, but thermal conductivity is about double that of Macor.