More than thirty different gems and ornamental stones are known to occur in Colorado. Amazonstone, amethyst, garnet, tourmaline, aquamarine, topaz, lapis lazuli, quartz crystal, smoky and rose quartz, sapphire, several varieties of agate, zircon, and other attractive stones are gathered within the state, mainly in the Mountain Province. Turquoise is known at several places in the volcanic area of southern Colorado. Alabaster is mined along the northeastern mountain front near Fort Collins and Loveland. Localities of interest to gem hunters are described in _Colorado Gem Trails and Mineral Guide_, by Richard M. Pearl. Gem Village, in southwestern Colorado on U. S. highway 160 between Durango and Pagosa Springs, is a favorite stopping place for tourists wishing to see or buy colorful and attractive Colorado stones such as petrified wood, agatized dinosaur bones, chalcedony, and jasper. WATER Although not all aspects of water and water supply are geologic, water is an important geologic agent, determining the shape of the surface, the distribution of minerals, and the location of caves. Water used in Colorado comes entirely from precipitation within the state, as all of Colorado’s rivers flow from Colorado outward toward the surrounding lower-elevation states. Surface Water [Illustration: A cross section through the Front Range northwest of Denver shows the redistribution and use of western slope water in eastern Colorado through the Colorado-Big Thompson Project. This project has cost about $160,000,000, but it is repaying the investment many times over by providing electric power and increasing farm production.] Moisture carried by prevailing westerly or northwesterly winds falls primarily on Colorado’s western slope, although at some times of year precipitation may come from the northeast or southeast. West of the continental divide, where population is sparse, there is a surplus of water. East of the divide, where more than 90 per cent of the population lives, water is in desperately short supply. The high and largely unpopulated Mountain Province receives by far the greatest proportion of precipitation, while agricultural areas of the Prairie and Plateau Provinces receive much less. Needless to say, the major problem involving water in Colorado is how to move it from areas where it is abundant to areas where it is needed. In many parts of the state, complex water laws and complicated irrigation canals and water systems were developed soon after the area became settled. Gradually but inevitably, water resources have been transferred from the western slope to the eastern. However, such transfer must be undertaken with due regard for the rights of downstream users, notably California, Arizona, and New Mexico.