ENGLISH GEM COLLECTION INFORMATION SHEET 9 (CASES 283-284) The new gem collection was begun in 2003 and currently contains 180 specimens, of which 160 are on display. Although the collection includes some precious gemstones (ruby, sapphire, emerald and aquamarine), most of them are collector's pieces; stones that are not often used in jewellery due to their low or medium hardness. There are 14 replicas of famous and historic specimens on display, as well as a collection of old cut gemstones and examples of the different stages involved in producing the brilliant cut, the most widely used technique with diamonds.

CASE 283 CASE 283 CASE 283 CASE 283 CASE 283 PRECIOUS OPAL AQUAMARINE BLUE TOPAZ SCAPOLITE TRAPICHE EMERALD

Opal is the most important gemstone Aquamarine is a blue gemstone variety Cut into a faceted pear shape, this The scapolite group forms an Emerald is a green gemstone variety within the silica group. The precious of beryl. Its blue colour is due to small specimen is a natural, untreated light isomorph series whose endmembers of beryl. The green colour is mainly variety of opal is the most highly amounts of iron (Fe2+) that replace blue colour. Unlike yellow, orange, are marialite (sodium endmember) due to small amounts of chromium, appreciated. It can be transparent, the beryllium in the crystalline brown, reddish or pink topazes, which and meionite (calcium endmember). although traces of iron Fe3+ and translucent or even opaque, and structure. However, when some of the contain many OH groups, the main The analyses carried out on this vanadium also play an important presents an optical effect termed a iron is present as Fe3+, a golden- group in this specimen is fluoride. brownish-yellow scapolite indicate a role. This magnificent specimen, cut “play of colour"” (not to be confused yellow hue is produced, rendering the Generally, reddish or pink topazes composition close to marialite. The into an oval cabochon, is unusual with opalescence, the milky effect seen aquamarine greenish-blue. This owe their colour to heat treatment, raw crystals show a prismatic habit because of its large size. It consists of in the common opal). On a microscopic specimen, cut into a faceted oval, while brown or deep blue specimens terminating in bipyramids with heavy a central hexagonal prismatic crystal level, precious opals are composed of contains hollow inclusions and fine are obtained by irradiation. Due to its striations along the prism faces (see surrounded by six other prismatic spheres of cristobalite and tridymite parallel tubes filled with fluid that recall appearance and colour, although not case 18 in the Systematic Mineral crystals. The hexagonal prisms are arranged in a lattice. The play of colour the rain. Aquamarines are considered its price, blue topaz can often be Collection). It presents refractive separated by black carbonaceous is produced by diffraction of incident the fifth most valuable gemstones. confused with aquamarine. However, indices of 1.548 and 1.560 and a matter. This rare variety of emerald is light passing through the layers of Many good quality aquamarines are it is easily identified by examining its density of 2.64 g/cm3. This gem has only found in Colombian mines and is microscopic spheres and the empty produced by heating greenish-yellow or physical properties. Topaz must be been given a faceted round fancy cut. highly prized by collectors. The name spaces between them. Scientifically, a brown beryls to 400-500ºC, which treated with great care since despite Its main feature is its size, and it is “trapiche” is of Spanish origin, and gemstone quality opal is called an opal converts ferric iron to ferrous iron, its high hardness (8 on the Mohs probably one of the largest faceted refers to the stone's resemblance to CT. There are five varieties of precious changing the colour to blue. There are scale), it has a tendency to fracture scapolites in the world. the mill machinery used to grind opal:thewhite,black,water,fireand no known synthetic aquamarines, but along the plane of the crystal due to its sugar cane. matrix opal. The oval cabochon glass and synthetic spinel imitations pronounced basal cleavage. This Na43924 Al Si O Cl / Ca 466243 Al Si O Co displayed in this case is a white opal are available on the market. means that any impact could crack it Malema (Nampula, Mozambique). Be32 Al (Si 618 0 ) with a very delicate play of colour or break it in two. Weight: 115,00 carats. La Peña, Muzo (Colombia). throughout the specimen. Be32 Al (Si 618 0 ). Dimensions: 30.60 x 23.65 mm. Weight: 29,21 carats.

Minas Gerais (Brazil). Al24 (SIO )(F,OH) 2 Dimensions: 23.29 x 20.35 x 8.72 mm.

SiO22 .nH 0 Weight: 47,12 carats. Brazil. Welo (Ethiopia). Dimensions: 29.10 x 19.00 x 14.40 mm. Weight: 150,01 carats. Weight: 11,69 carats. Dimensions:42.30x29.90x17.70mm. Geominero Dimensions: 26.80 x 15.40 x 5.60 mm. eo Instituto Geológico y Minero de España ENGLISH GEM COLLECTION INFORMATION SHEET 9 (CASES 283-284)

CASE 284 CASE 284 CASE 284 CASE 284 CASE 284 SMOKY QUARTZ AMETRINE QUARTZ LEMON QUARTZ YELLOW ORTHOCLASE GREEN FLUORITE

This variety of quartz ranges from This is a variety of bicolour quartz Lemon quartz obtains its colour Orthose or orthoclase is a feldspar Fluorite is a very beautiful mineral yellowish-brown to almost black in incorporating purple amethyst and through an artificial gamma-ray mineral, and is a common constituent due to its colour and transparency. colour. The darker variety is called yellow citrine, hence the composite irradiation process followed by a mild of many rocks, but has little value as a However, it is unsuitable for use in morion, while the yellowish-brown name of ametrine. As it is only mined heat treatment. Not all quartz can be gem. However, some transparent jewellery because of its low hardness variety has been called cairngorm, in one place, the Anahi mine in treated this way to obtain lemon varieties can be used as gemstones. (4 on the Mohs scale) and is only after the Scottish mountain where it German Busch, Santa Cruz (Bolivia), quartz. The most commonly used Recently, it has been found that the used as a collection piece. Fluorite was found in the 19th century. This very close to the border between type is milky quartz from a very specimens obtained in Madagascar comes in a wide range of colours: specimen, cut into a faceted oval in Brazil and Uruguay, it is also known by specific area in Brazil, Sao José de are actually sanidine, a type of colourless, yellow, brown, green, Jaipur (India), is valuable because of the trade namebolivianite . Legend Zafira y Cristalina. This specimen is sodium-potassium feldspar, although blue, violet or red, and some its large size and natural colour. The tells that this stone has been known notable for its large size and the in the trade these gems are still called specimens even change colour colour is due to structural changes since the time of the Spanish quality of its fancy oval cut. The gem yellow orthoclase. This transparent, according to the light. From a caused by natural radioactivity and conquistadors (17th century), but was cut using a novel technique in pale yellow variety owes its colour to gemmological viewpoint, the most also the presence of aluminium, there is no reliable information about which the facets are concave rather traces of ferric iron, and comes from attractive varieties for faceting are which partly replaces the silicon. The it until the 20th century. Mass mining than flat. Although the distribution is the pegmatites of Itrongay, those presenting an emerald green intensity of the colour depends on the of the deposit (illegal and without the same as in a traditional cut, Madagascar. The specimen presents colour. It is difficult to obtain large amount of radioactivity the stone has studies or planning) began at the end concave facets produce much more a very strong trichroism (deep orange- faceted stones because this mineral received. The colour of much of the of the 1970s. However, the first brilliance than classic cuts. yellow-orange) and inclusions in the presents perfect cleavage according material on the market is obtained by systematic studies began in 1989, form of short needles. It has a faceted to the faces of the octahedron. This artificial irradiation. Better quality and the national mining company SiO2 round fancy cut. fluorite specimen has been cut into a gems may come from hydrothermal Minas y Metales del Oriente started Brazil. pear shape using a technique that deposits in the Swiss Alps; however, to develop mining technology in the Weight: 229,65 carats. K(AlSi38 O ) produces concave facets (similar to most gems nowadays come from area. Specimen in an octagonal Dimensions: 50.50x38.30x23.70 mm. Itrongay, (Madagascar, Africa). that used for the lemon quartz). Brazilian pegmatites. emerald cut. Weight: 48,60 carats. Dimensions: 24.69x24.95x15.64 mm. CaF

SiO2 SiO2 China. Brazil. German Busch, Santa Cruz (Bolivia). Weight: 64,94 carats. Weight: 1.781,00 carats. Weight: 59,04 carats. Dimensions: 30.80x22.70x16.00 mm. Dimensions: 98.20 x 66.50 x 47.60 mm. Dimensions: 25.74 x 23.87 x 11.95 mm.