Nebraska State Gemstone
Chalcedony is a catchall term that includes gemstones with microscopic crystal structures composed of quartz, many also containing the mineral mogánite. Chalcedony is transparent with a waxy luster. When these gemstones are concentrically banded, they are called agates. Blue chalcedony is commonly known as blue agate. Chalcedony is a silicon dioxide with iron and aluminum oxides sometimes present. Iron, nickel, copper, and other impurities cause the color variations.
Found in northwestern Nebraska, blue chalcedony is a pale-colored stone. It sometimes has a dark internal form with bands of blue and white and a colorless streak. Blue chalcedony was designated the Nebraska state gem on the state centennial in 1967.
The term chalcedony is derived from the name of the ancient Greek town Chalkedon (Chalcedon in English) in Asia Minor, which today is the Kadıköy district of Istanbul, Turkey.
The word agate comes from the site where the stone was originally reported, the Achates River, or Αχάτης in Greek, which is now known as the Dirillo River and is located in Sicily, Italy. The Greek naturalist and philosopher Theophrastus discovered agates sometime between the 4th and 3rd centuries BCE. Agates were widely used in the ancient world.
FORMATION AND OCCURRENCE
Agates are normally associated with volcanic lava rocks and are also found in some metamorphic rocks. Silica-rich water percolates through the rock, escaping as gas and depositing traces of the silica in the rock’s crevices. Layers are formed, and eventually the rounded nodules called agates result.
Blue agate has been found in wind-deposited claystones and silt in the Chadron Formation of the Oligocene Age in Sioux and Dawes Counties in Nebraska. The chalcedony probably originated from silica that was freed when devitrification (changing from a glassy to a crystalline state) of wind-blown volcanic ash took place. The chalcedony appears to have formed in or near sources of alkaline water.
Blue chalcedony is found in the U.S. states of California, Nevada, Oregon, and Nebraska, as well as in Namibia.
Chalcedony is widespread, occurring in most areas of the world. In addition to the site where the mineral was first reported in Sicily, major chalcedony locations include Argentina, Botswana, Brazil, Germany, Mexico, Morocco, Poland, the U.S., and other regions of Italy. The important gem region of Idar-Oberstein, in the Hunsrück Mountains in Rheinland-Pfalz, Germany, got its start as a source for agate thousands of years ago.
Because of its abundance, durability, and beauty, chalcedony was one of the earliest raw materials used by humans. The earliest recorded use was for projectile points, knives, tools, and containers such as cups and bowls. Early humans made weapons and tools from many varieties of chalcedony including agate, agatized coral, flint, jasper, and petrified wood. Agate, along with petrified wood, was then elevated from functional use as tools, vessels, and weapons to gem status, being used for decorative and religious purposes.
From antiquity through the Renaissance period, collecting bowls made of agate was a popular hobby. In fact, this hobby is what led to the growth of the gemstone industry in the Idar-Oberstein district of Germany, one of the world’s leading centers for gemstone cutting and trading.
Today, agate is chiefly polished for use in ornamental applications such as in brooches and pins, pendants and charms, beads, mosaics, and dream catchers. Agate is one of the most varied and most popular gemstones. Blue agate in particular is a popular gemstone used in jewelry. Agate also has more industrial applications, including inkstands, mortars and pestles, and letter openers.
Agate has been believed for centuries to protect the wearer or holder of the stone from a wide variety of dangers and to bring good fortune. Because it has been known and used by humans for millennia, the supposed benefits of agate are innumerable and cross into many cultures.
Agate is the mystical birthstone for September and the birthstone for the zodiac sign of Gemini. Agate is given for the 12th and 14th wedding anniversaries.
Chemical Formula: SiO2
Crystal Structure: Trigonal
Hardness (Mohs): 7; lower in impure varieties
Color: Clear (in pure form)
Birefringence: +0.009 (B-G interval)
Refractive Index: 1.544-1.553 - Dr +0.009 (B-G interval)
Density: 2.65 constant; variable in impure varieties
Some data courtesy of the Mineralogical Society of America
Author: World Trade Press