I don't collect minerals, so I have very few. This synthetic corundum rod was used in a hospital laboratory.
They're called "sapphires" despite the fact that they are pure Al2O3. They are scratch resistant, not reactive, can withstand extreme heat, and have special optical and mechanical properties.
A sapphire rod just a few inches long can be 2000 C at one end and 20 C at the other, allowing tests at a wide range of temperature simultaneously using only one part.
Also, check out this extremely retro tube that I keep my synthetic sapphire in. My dad gave me this sapphire rod in 1978. It came from his lab, but it broke and had to be replaced.
Still got a few hours left, so let me add that corundum, as industrial sapphire, has many uses in laboratory medicine.
It's scratch resistant, heat resistant, acid proof; 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.
And cuvettes? Totally reusable.
At this time, I see only a 10 point difference between the contestants, so here is my last bit of corundum content.
This is #syenite, an igneous rock, which is kind of like granite, but not really.
Corundum sometimes occurs naturally in syenite, but obviously not in my sample.
David Michael Self