Tabular slabs of porphyritic hornblende andesite from Elephant Mountain were shaped into aboriginal milling stones and pestles and carried to living and processing locations in the Mojave River region. The milling-implement quarry at Elephant Mountain, first described by Nuez in 1819, was identified archaeologically and subsequently studied. Worked slabs of andesite, broken and discarded preforms, andesite debitage, and hammerstone quarrying and production tools characterize the archaeological deposit at the quarry. Production technology differs somewhat from that employed at other milling-implement quarries. It is proposed that the long-term retention of the abrasive quality of the Elephant Mountain stone made it especially useful for milling. Milling stones from the Hinkley site, about 15 mi. upstream on the Mojave River, originated at Elephant Mountain, as demonstrated by thin-section petrography. Certain lines of evidence suggest that the Elephant Mountain stone may have been exploited as early as 3,500 B.P., that it may have been used by groups expressing or influenced by material culture traits of the Lower Colorado River region, and that the quarry may not be unique in the region. Sourcing of milling implements to their quarry has the potential to add to our understanding of regional prehistoric economic and social networks.