What is it?

[jslo]

Big ass magnet the way that pen is sticking to it

[fitz19d]

That an actual thread on it or just ridges.

[Duman]

Godzilla's butt plug?

[GilpinGuy]

That an actual thread on it or just ridges.

If it's threaded maybe it's a plug for a well bore?

[encorehunter]

It has opposing threads. Magnets do not stick to it. I may have an answer, but I'll let this go on for a little while. The threads chip with a pocket knife.

[Jamnanc]

Magnesium. Don't set it on fire.

[ray1970]

Magnesium. Don't set it on fire.

Ooo. If it is magnesium, we need a video when you do burn it.

[Gman]

My guess is an aluminum ingot. Ingots often have a trapezoidal shape which makes them easier to remove from the mold.

[Jamnanc]

Must be a weird sort of internal coupler for large diameter threaded pipe. Probably aluminum. I'm struggling to figure out how it would be removed though.

[BushMasterBoy]

Ok, I think I got it. Graphite mold piece. Graphite crucibles are quite common. Here is a pic of one I heated to about 2300 degrees F