Quartz, minerals, coal cost the same to produce and they have same production rates, you can include iron ore too, but it is slightly more expansive
market status:
Minerals: one page of offers, cheapest for 11.40 but then it rises to 20+ (i don't think the 20+ will sell quicky, but there is not much of the cheaper ones)
Coal: 4 pages of offers, but this market moves quicky, cheapest offer @ 9.25c and then many offers @ 10c






3 pages of offers, first page ends @ 5c


cheapest offer is 2c, and there are huge amounts of quartz for lower than 6c
now, why would you produce something that you can sell for 2-3c, if you can produce something else with the same expenses in the same building that you can sell for 10c ?