Tremendous beryl crystals weighing as much as several tons, but not of gem quality, have been discovered in a few localities. Large crystals of gem quality also occur in nature, and large cut stones of aquamarine and other colors of beryl are relatively common. Among the fine examples of beryl in the National Gem Collection is a remarkably large (2054-carat), flawless cut stone of rich yellow-green. This gem and others in the collection weighing 1363 carats, 1000 carats, 914 carats, and 578 carats accentuate the occurrence of large gem crystals of beryl in Brazil. [Illustration: Four large cut stones, all from Brazil, illustrate the color range of beryl. Top, a 578-carat green beryl; left, a 235-carat morganite, gift of Mr. and Mrs. Frank Ix, Jr.; bottom, a 133-carat gold beryl; and, right, a 187-carat aquamarine. (Half actual size.)] The finest emeralds are not found in pegmatites. At Muzo in Colombia, the most prolific source of the finest emeralds, they occur in veins with calcite, quartz, dolomite, and pyrite. The veins cut through dark-colored, carbonaceous limestone and shale. Mining at Muzo began 350 years ago and still continues sporadically to meet market requirements. Russian emeralds occur as good-sized crystals in mica schist, a metamorphic rock. They occur there with chrysoberyl, phenakite, and common beryl. Some of the smaller stones have good color and have been cut into valuable gems. Brazil, which produces many extraordinary aquamarines and other beryls, has not produced quality emeralds. Periodically, over the centuries, there have been reports of new discoveries of emerald, but so far none of these has begun to rival the Muzo source in either quantity or quality of the gems produced. [Illustration: This tremendous golden beryl from Brazil, weighing 2054 carats, is the largest cut beryl known of this color. Cut stones of this size that contain no visible flaws or inclusions are most unusual. (Three-fifths actual size.)] Although Brazil supplies the finest aquamarine and Colombia the finest emerald, several localities in the United States are sources of good-quality beryl of these colors. Foremost among these localities are Maine, California, and Connecticut for aquamarine and North Carolina for emerald. Morganite of pale pink to deep peach color, from California, is also notable. Various New England mines in Maine, New Hampshire, and Connecticut and the gem mines of the Pala and Mesa Grande districts of California have produced other colors of gem beryl. However, most of the beryl mined in the United States is used as an ore for beryllium, as little of it is of gem quality. Because of its hardness (about 8), vitreous luster, beautiful color, and rarity, emerald always has been highly prized as a gem. Fine-quality emeralds may be more costly than fine diamonds. Other kinds of beryl have the same physical properties as emerald, but since they are less rare their relative value is lower.