I just picked up (not one but) TWO soapstone makers with 12 refills for a whopping $7.28 on Amazon. Marks white on the tube, unaffected by heat, wipes off clean when cooled. Includes a nifty clip so you can nerd out with your pocket protector. Made in China, so get 'em before the tariffs hit! 

https://www.amazon.com/dp/B0BRSTFK4M?psc=1&ref=ppx_pop_dt_b_product_details

Ooo, these look neat! Soapstone is my favorite for clear or darker colored glass. Let us know if they slip/ rotate in the grip? I have opted for the square soapstones sharpened in the bench grinder but havent yet found (or searched much) for a holder that doesnt slip. Then I end up using broken bits of the rectangle stone... sometimes with tape wrapped around them for quick visibility on the table.

They are gonna have to sand off my marks :) Get out your 1500 grit

The grip could be a little tighter, but it works for marking perpendicular to the axis of the holder. Its probably not tight enough to drag across a rusty piece of flat steel, so maybe that explains the price. A piece of paper wrapped around the soapstone stick might be a fix, but I'm too lazy to try that until it slips enough to affect my language.

I made several marks on a piece of 10mm clear and then punished the center mark in the crossfire until the tube deformed. Once cooled all the marks wiped clean with a paper towel. The hottest mark stayed white in the flame and took a little bit more rubbing with the paper towel. No sanding needed.