It's totally worthless once you get the quest done -- I ended up with tons of this because even though I talked to Varun in Windurst during the Rock Racketeer quest (when I should have), the mythril seam REFUSES to give me a "sharp stone". All I have is tons of mine gravel and I lost 12 picks. Don't waste time gathering mine gravel -- just cut your losses and get rid of it if you make the mistake that I did. I'm not sure why so many people BUY mine gravel at the AH, unless it's used for a recipe (in which case it needs to be added here)...
Well, because it's easier to buy the mine Gravel instead of having to mine it yourself when you are doing the Rank 3 Mission, and your first stop is Bastok.
I found this mining amythril seam yesterday. I have no idea if it's good for anything or not, or maybe is a precursor for one of the upcoming quests. I was wondering around the pal mines and ran across the machine on the 3rd floor. Interested I opened it and it said there were traces of mine gravel inside, so of course i put it in. Pulled the lever next to it, hopped down to the second floor and pulled the lever there. Out comes mythril sand, it doesn't sell for very much so there must be some quest somewhere that requires it.