I posted this last year, but here it is again:
This is a synthetic corundum rod from a hospital laboratory. These are called "industrial sapphire" despite the fact that they are pure Al2O3. Scratch resistant and non-reactive, a sapphire rod only a few cm long can be 2000C at one end and 20 C at the other, so you can test a wide temperature range with just one part.
Corundum (as industrial sapphire) is not only scratch resistant, heat resistant, and acid proof; It can be a rod, tube, lens, or bearing. The only material that even comes close is diamond, but diamond is too brittle to make tubes.
You can even make tiny square vials called "cuvettes", which allow your sample to be heated to 1000s of degrees for spectral analysis.
(Most cuvettes are made of quartz, glass, or plastic, but for extreme tests, sapphire is sometimes used.)
David Michael Self
Mineral Cup & Ore Cup