Spent many years using square soap stone for pipe and boiler work but sometimes we would work with material that soap stone is no good for. So Ive all ways kept fine tip markers (black), #2 pencils (for drawing ISOs), carpenter pencils, good ink pens and pretty much any other writing instrument i could find which would leave a precise marking. Kept an old baby wipe box mostly full of various types of writing instruments. I guess it depends on what im working with as to what is my favorite type of writing instrument.
Everything i build comes with a redneck warranty. If it breaks you get to keep both pieces.