• Abstract and Figures
• Public Full-text

@tftrs1//8Lhmdq`knesgdLnmsg9Bgqxrnoq`rd We’re happy to be featuring chrysoprase, the rarest and most valuable of the gem varieties of chalcedony. Our specimens were collected at a recently reopened gemstone mine in Australia’s historic goldfields. Our write-up explains the origin of chrysoprase’s beautiful, apple-green color as well as the fascinating gold- mining history of Western Australia–it was the search for gold that lead to finding chrysoprase! OVERVIEW PHYSICAL PROPERTIES Chemistry: SiO2 Silicon Dioxide, with small amounts of nickel. Class: Silicates Subclass: Tectosilicates Group: Quartz Subgroup: Microcrystalline Quartz (Chalcedony) Crystal System: Hexagonal Crystal Habits: Microcrystalline, occurs in compact form as veins and nodules Color: Green to apple-green, occasionally yellowish-green Luster: Waxy and vitreous to dull Transparency: Translucent to nearly opaque Streak: White Refractive Index: 1.530-1.539 Cleavage: None Fracture: Conchoidal to subconchoidal and irregular, brittle to tough Hardness: Mohs 6.5-7.0 Specific Gravity: 2.58-2.64 Luminescence: None Distinctive Features & Tests: Best field marks are translucency, green color, hardness, and occurrence in nickel-rich environments. Can be confused with green varieties of smithsonite [ZnCO3], prehnite [Ca2Al2Si3O10(OH)2], and jade [jadeite, Na(Al,Fe)Si2O6]. Dana Classification Number: 75.1.3.1 NAME “Chrysoprase,” pronounced KRIH-sah-praze, is derived from the Greek chrysoprasos (chrysos, “gold,” and prason, “leek”), which translates literally as “golden leek.” Although the name was originally used for certain yellow gemstones, chrysoprase now refers exclusively to the green variety of chalcedony. Other names for chrysoprase include “Australian jade,” “Australian imperial jade,” “quartz jade,” “green quartz,” “chrysophrase,” “prase,” “chrysoprasus,” “green hornstone,” and “jadine.” In European mineralogical and gemological literature, chrysoprase appears as crisoprasa and crisoprasio. COMPOSITION: Chrysoprase is the nickel-rich, green variety of chalcedony, or microcrystalline quartz. Quartz, chemical formula SiO2, contains two elements, the semimetal silicon (Si) and oxygen (O). The molecular weight of quartz is made up of 53.26 percent oxygen and 46.74 percent silicon. Quartz is a member of the silicates, the largest of all mineral classes. The fundamental building block of the silicates 4- is the silica tetrahedron (SiO4) , which consists of a silicon ion surrounded by four equally spaced oxygen ions positioned at the four corners of a tetrahedron (a four-faced polyhedron). Chrysoprase forms from the weathering of ultramafic (iron- and magnesium-rich) serpentinite rocks that oxidize in a process called laterization to produce laterite (pronounced LATE-er-ite), a residual, crumbly, reddish rock or earth consisting of an indefinite mixture of oxides, hydroxides, and hydrous oxides of iron or aluminum. Silica and nickel ions carried by groundwater then circulate through the laterite where the chemical environment causes them to precipitate simultaneously as chrysoprase in the form of veins and nodules. Bnoxqhfgs 1//8 ax Qhbg`qc % Bgdqxk Rhsshmfdq L hmdq`k ne sgd L nmsg Bkt a 0 6 6 / N qu hkkd @ u dmt d B`l aqh`+ B@ 82 3 17 0 ,7 //,83 0 ,4 4 83 v v v -l hmdq`knesgdl nmsgbkt a-nqf 1 @tftrs1//8Lhmdq`knesgdLnmsg9Bgqxrnoq`rd COLLECTING LOCALITIES: Since the 1960s, Australia has produced most of the world’s chrysoprase. Important sources include the Yerilla Chrysoprase Mine at Yerilla in Menzies Shire, Western Australia; the Marlborough chrysoprase deposit near Rockhampton in Queensland; Hanging Rock at Nundle in Parry County, New South Wales; and Mount Davies near Pipalyatjara in South Australia. Other sources include the historic mines at Szklary near Zbkowice –lÅskiein in Lower Silesia (Doino—lÅskie) Poland; Revdinsk near Ekaterinsburg in Russia’s southern Urals; the Diethensdorf Quarry at Chemnitz in Saxony, Germany; Hanety Hill at Dodoma in the Central Region of Tanzania; and Saryku-Boldy near Karkaralinsk in Qaraghandy Oblysy (Karaganda Oblast’), Kazakhstan. In Japan, chrysoprase cobbles are found on Oyashirazu Beach at Ohmi in Niigata Prefecture in the Chubu Region of Honshu Island. In the United States, most chrysoprase is collected in Arizona, where important sources include the Old Dominion Mine at Globe in the Globe district and the Inspiration Mine at Inspiration in the Miami-Inspiration district, both in Gila County; the western slopes of the River Mountains in the Weaver district in Mohave County; and the Perry Chrysoprase Mine in the Plamosa district of La Paz County. California localities include the Calaveras Asbestos Mine in Calaveras County and the Visalia Chrysoprase Mine at Visalia in Tulare County. Chrysoprase also occurs in Colorado, Oregon, Rhode Island, and Vermont. HISTORY, LORE, & USES: Because of its rarity, soft translucency, rich green color, and workability, chrysoprase has been valued as a gemstone and a decorative stone since antiquity. Today, chrysoprase is fashioned into cabochons and beads for use in rings, pendants, necklaces, bracelets, and brooches, and carved into a variety of figurines and decorative objects. Top grades of gem-quality chrysoprase have a clean, uniform, vivid, apple-green color, soft translucency, and few visible inclusions. As the rarest and most costly of the chalcedony gemstones, the finest grades of chrysoprase are often sold on a carat basis. Top-quality chrysoprase in finished cabochons is valued as high as $5 per carat, meaning a large cabochon weighing one-half ounce (64 carats) can cost $300. Metaphysical practitioners believe that chrysoprase imparts a sense of grace that facilitates meditation, promotes general healing, encourages open-mindedness and acceptance of one’s self and others, prevents depression, and protects against anguish, nightmares, and bad influences ABOUT OUR SPECIMENS: Our specimens were collected in Australia at the recently reopened Yerilla Chrysoprase Mine at Yerilla in Menzies Shire in the state of Western Australia. Yerilla, a historic, abandoned gold-mining town in the remote outback of Western Australia, is 400 air miles east-northeast of the coastal city of Perth. Chrysoprase was discovered at Yerilla in the 1980s. Mining was initially conducted from 1992 to 1995. The mine then became inactive until it was sold and new owners reopened it in 2007. Mining at the Yerilla chrysoprase deposit is now conducted by mechanized, surface trenching to a depth of about 25 feet. When the chrysoprase-bearing host rock is exposed, miners visually inspect it for chrysoprase veins and nodules, extract them mechanically, then sort the stones by hand. The cause of chrysoprase’s lush green color is explained in the last paragraph under “Composition” in the comprehensive portion of the write-up. The color of chrysoprase sometimes fades upon exposure to prolonged high heat or intense sunlight, so avoid both when displaying your piece. Gemologists believe that the layers of willemseite platelets in chrysoprase absorb water and that color fading is caused by loss of this water when the stone is heated. Color intensity can be restored by storing the chrysoprase in dark, humid, cool conditions to replace the lost water. Light or thermal energy that displaces the chromophoric nickel ions from their positions within the crystal lattice can also contribute to color fading. 10 YEARS AGO IN OUR CLUB: Thenardite, Soda Lake, Carrizo Plain, San Luis Obispo County, California, a rare evaporate mineral named for French chemist Baron Louis Jacques Thenard (1777- 1857). This is the only mineral we’ve featured from our home county! We moved into our present home on Orville Ave in Cambria in August 1999, after moving to Quartz Hill in 1991 and to Cambria in 1995. Bnoxqhfgs 1//8 ax Qhbg`qc % Bgdqxk Rhsshmfdq L hmdq`k ne sgd L nmsg Bkt a 0 6 6 / N qu hkkd @ u dmt d B`l aqh`+ B@ 82 3 17 0 ,7 //,83 0 ,4 4 83 v v v -l hmdq`knesgdl nmsgbkt a-nqf 2 @tftrs1//8Lhmdq`knesgdLnmsg9Bgqxrnoq`rd COMPREHENSIVE WRITE-UP COMPOSITION As you can imagine, it is always exciting to feature an especially beautiful mineral like chrysoprase! We were used to seeing small amounts in the stock of our Australian suppliers over the years, until last year in Tucson when one of our sources had a huge amount! Of course by then, it had been picked over, so we made arrangements with him to see what was new this year in Quartzsite, and hand pick your pieces before all the best were gone, and the rest, as they say, is history! Prior to mailing out your specimen, we had one of the most impressive lots of gorgeous chrysoprase ever! Quartz occurs in both macrocrystalline and microcrystalline forms. Macrocrystalline quartz exhibits large, transparent-to-translucent, individual crystals or groups of crystals and includes such varieties as amethyst, rock crystal, milky quartz, smoky quartz, and rose quartz. Microcrystalline quartz or chalcedony, which is compact and consists of interlocked, microscopic silica grains or fibrous silica crystals, includes such varieties as agate and jasper. Chrysoprase is a rare, green color variety of chalcedony. For an in- depth discussion of how the microcrystalline and macrocrystalline forms of quartz differ, see “Microcrystalline Quartz” in our August 2008 write-up on Mookaite jasper. Contact us if you need a copy. As shown by its chemical formula SiO2, quartz contains two elements, the semimetal silicon (Si) and oxygen (O). The molecular weight of quartz is made up of 53.26 percent oxygen and 46.74 percent silicon. The cation (positively charged ion) in the quartz molecule is the silicon ion Si4+ with a +4 charge. The anion (negatively charged ion) consists of two oxygen ions 2O2- with a collective -4 charge. The balance of these +4 cationic and -4 anionic charges provides the quartz molecule with electrical stability. Quartz is a member of the silicates, the largest and most abundant of all mineral classes. The silica 4- tetrahedron (SiO4) , the fundamental building block of the silicates, consists of a silicon ion surrounded by four equally spaced oxygen ions positioned at the four corners of a tetrahedron (a four-faced polyhedron).

Load more

  10

Copy Link