How much carbon is produced in transporting my stone?

Figure that stone will average about 165 lbs/cubic feet.
Figure that your stone will weigh about 13.75 lbs/SF/inch of thickness

CO2 Grams/ton/mile are:
Truck = 176.74
Rail = 70.10
Ship = 19.97 (YMSW uses oceanic transport wherever possible.)

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Source:
European Environment Agency TERM 2003 27 EEA 31

Organization for Economic Cooperation and Development (1997), "The Environmental Effects of Freight" Table 5, 9, 14.
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One gallon of gasoline produces about 20 pounds of CO2