Unfazed by slumping oil prices and battering in the stock market, firms that supply sand and guar gum for shale oil and gas companies are not ready yet to call an end to a four-year boom spurred by hydraulic fracturing technology. Perhaps, but the optimism could also reflect confidence that the U.S. Oil prices have fallen 30 percent since late June and shares of such firms as U.S. Silica Holdings and Hi Crush, which supply sand to U.S.