The value of nonfuel minerals produced in North Dakota in 2001 was estimated at about $39 million, up about 12% from 2000. Construction sand and gravel accounted for more than 70% of the value ($27.6 million) of North Dakota's nonfuel mineral output, from a production of 10.6 million metric tons. Recovered elemental sulfur is the 2nd most important mineral produced in North Dakota, in terms of value. Sulfur and other byproducts such as krypton, xenon, anhydrous ammonia, and liquid nitrogen are recovered during natural gas processing at five plants in the western part of the state. Lapidary and collectible materials such as petrified wood, agates, jasper, and flint are also found in North Dakota.