During World War II, engineers perfected the little glass balls to such a degree that they could be substituted for steel bearings. Tons of these glass balls go to lithographers and engravers, to be used in smoothing the surface of copper printing plates.
In the oil fields, refineries use acid-proof marbles as filters and condensers. The glass balls are injected into old oil wells to prepare them for possible further use. Consequently, millions of dollars worth of oil can now be recovered.
Some fish hatcheries put marbles in the bottom of pools for better results at spawning. Paper mills use glass balls in manufacturing. Recently has it been found out that marbles are, highly useful in the spinning of glass thread.
Some time in 1948, a telegram from Johannesburg, South Africa, reached a marble manufacturing factory in the US requesting a shipment of 100,000 marbles in "three-colour strip combinations on opalescent glass." They were to be used as currency, but were captured by warring tribes. The mibs had to be re-minted.
(I've been reading a lot of marble articles) ;-)