GEMSTONES. Alessandro Grippo, Ph.D. Carbonate Gemstones collection. Natural History Museum, Denver, Colorado Alessandro Grippo

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1 GEMSTONES Alessandro Grippo, Ph.D. Carbonate Gemstones collection Natural History Museum, Denver, Colorado Alessandro Grippo

2 What is a Gemstone? There is no official definition A gemstone is any inorganic, or organic, or synthetic material that is pleasant to humans Minerals (e.g., diamond) Mineral aggregates (e.g., lapis lazuli) Rocks (e.g., onyx marble) Organic materials (e.g., amber, ivory, fossils) Man-made materials (synthetic substances)

3 Gemstones can be appreciated for their: Color Special optical properties Luster Hardness Inclusions Rarity

4 A gemstone is usually prepared through cutting and polishing Otherwise insignificant raw materials can then be turned into gemstones Left: kimberlite, a diamond ore. Right: rough diamonds extracted from kimberlite NWT Diamond Center, Yellowknife, Northwestern Territories, Canada Alessandro Grippo

5 Progressive stages of diamond cutting and polishing NWT Diamond Center, Yellowknife, Northwestern Territories, Canada Alessandro Grippo

6 Gemstones and their influence Gems have intrigued humans for at least 10,000 years In the past, they were reserved for the wealthy, and served as status symbols These can now be seen in many museums and treasure-vaults The Iron Crown of Lombardy (ca. 524, gold, silver, garnets, sapphires, amethysts) Monza, Italy Christian IV s Danish Crown (1595, diamonds and pearls) Rosenborg Castle, Copenhagen, Denmark

7 Gemstones and their influence Because of lack of knowledge in the past, gems were often seen as mysterious objects Amulets, talismans, spiritual meanings Remedies against illnesses Today, gems are mostly appreciated for their beauty and characteristics, and their commercial value As a capital investment, of all gemstones only diamonds are really suitable

8 Classification of Gemstones Mineral gemstones are classified in 10 major classes, according to their crystal chemical composition Elements Sulfides and Sulfosalts Halides Oxides and hydroxides Carbonates and nitrates Borates Sulfates & others Phosphates & others Silicates Organic Substances

9 Classification of Gemstones Commercially we distinguish: Best-Known Gemstones Traditionally traded from ancient times Lesser-Known Gemstones These used to be collector items, but are more popular today Gemstones for Collectors Of no particular use in jewelry: too soft, brittle, endangered, rare Rocks as Gemstones Organic Gemstones Materials that, despite their organic origin, have preserved or acquired a certain stone character Imitation and Synthetic Gemstones

10 Value of Gemstones There are no firm prices for gemstones: supply and demand regulate the market There are big differences between Diamonds and socalled Colored Stones Diamonds value-determining elements (such as color, purity, and cut) are internationally defined and controlled Colored Stones have no set trade rules and values can fluctuate. In any case, prices per carat* for Colored Stones are much less than those for Diamonds *a carat is equivalent to 0.2 grams

11 Precious Gemstones transparent, with an even coloration, and a hardness greater than quartz (7) Diamond C Usually clear, appeal comes from their brilliant luster Form at high P and T in Earth s interior Corundum Al2O3 (varieties Ruby and Sapphire) Red in ruby comes from Cr impurities Blue in sapphire comes from Ti and Fe impurities Form in metamorphic& igneous rocks Mined from sedimentary deposits Beryl Al2Be3(Si6O18) (varieties Emerald and Aquamarine) Green in emerald comes from Cr impurities Blue in aquamarine comes from Fe impurities Found in pegmatites

12 Other common best-known Gemstones Spinel Quartz Rhodochrosite Topaz Jade Lapis Lazuli Garnet Peridot Azurite Zircon Hematite Malachite Tourmaline Pyrite Opal Spodumene Turquoise

13 Lesser-known Gemstones Cassiterite Epidote Apatite Fluorite Sphalerite Cerussite Chrysocolla Serpentine Rocks as Gemstones Onyx-marble Tufa Orbicular Diorite Obsidian Alabaster Fossils (petrified wood and other fossils)

14 Organic Gemstones Coral Calcium carbonate skeletal structure Jet A form of fossilized wood Amber Endured tree resin Pearl Calcium carbonate coatings within mollusks shells Mother-of-Pearl Calcium carbonate inner coating of mollusks shells

15 Pegmatites and Gemstones Pegmatites are extremely coarse-grained intrusive igneous rocks Most pegmatites are granitic

16 Some pegmatites are mined for gemstones Emerald & aquamarine occur in pegmatites crystallized from a solution containing the element Be A lot of rare minerals are only known from pegmatites; these minerals are mainly of interest to collectors and museums Above: Emeralds in pegmatitic granite Below: Topaz in Na-Plagioclase

17 Gemstones in metamorphic rocks Ruby in a marble matrix Luc Yen, Vietnam GIA:

18 Gemstones in sedimentary rocks Red crystals of Rhodochrosite in a sedimentary matrix of clear Quartz, black Sphalerite, purple Fluorite, pale yellow Calcite, and brassy Chalcopyrite Sweet Home Mine, Alma, Park County, Colorado Natural History Museum, Denver, Colorado Alessandro Grippo

19 What about Gold? Colorado gold nuggets collection Natural History Museum, Denver, Colorado Alessandro Grippo

20 Gold and other metals Gold (Au), Silver (Ag), Copper (Cu) are native elements and precious metals, not gemstones Gold is commonly associated to felsic intrusive igneous rocks and can be found in veins and in pegmatites Upon weathering, gold can show up in sedimentary deposits in nuggets of all sizes