Gems

Gems

Gems

Peridot

Peridot, or chrysolite, is a variety of forsterite. Its coloration is usually between the shades of emerald green or light green, but it can present variations between greenish yellow, greenish brown or brown.

Peridot crystals have been explored at least since the beginning of the first millennium BC on the small island of Zabargad in the Red Sea near the Ras Banas peninsula. The island was named Topazo. The peridot deposit of Zabargad is historically the most famous and most important.

Peridot is one of the few gemstones that exist in just one colour, its agent of colour is iron. It is important to note that there is no known treatment that can improve the quality of this stone.

Peridot deposits of singular quality can be found in Minas Gerais, in the south-eastern region of Brazil.

Gemmological aspects:

  • Variety: Peridot
  • Group: Olivine
  • Colour: from yellowish green to greenish yellow to brownish green
  • Chemical composition: (Mg, Fe) 2SiO4 – iron and magnesium
  • Hardness: 6.5—7 (Mohs scale)
  • Specific weight: 3.34 (+0.14, -0.07)
  • Refractive index: 1.654—1.690 (± 0.020)
  • Fluorescence: inert / none

Topaz

Topaz is historically considered one of the most important stones, as well as being an excellent stone for jewellers due to its high refractive index, hardness (8 – Mohs scale) and almost no sensitivity to chemicals.

Its origin and denomination are probably related to the Island of Topazos, situated in the Red Sea, and which is now known as Zabargard, an ancient deposit of peridot.

In the Middle Ages, the colourless and yellow topazes of Schneckenstein (Germany) were known, but the great popularity of this gem was later recorded in the first half of the 18th century when the important topaz deposits of the Ouro Preto region of Minas Gerais, Brazil, were discovered.

Among the topazes of Ouro Preto were yellow, orange, pink and red varieties, the latter scarce and commercially nicknamed imperial topaz. At present, it is still only in Ouro Preto that commercial quantities of yellow, orange or imperial topaz are produced.

Gemmological aspects:

  • Variety: Imperial topaz and cat-eye topaz
  • Group: Topaz
  • Colour: colourless, yellow, orange, brown, pink to red to red-violet; Blue: light to dark, light green
  • Chemical composition: Al2 (F, OH) – aluminium silicate with fluorine content
  • Hardness: 8 (Mohs scale)
  • Specific weight: 3.53 (± 0.04)
  • Refractive index: 1.619—1.627 (± 0.010)
  • Fluorescence: yellow to brown and pink to red – weak to moderate. Pink – moderate. Blue and colourless – from inert to weak.

Tourmaline

The name tourmaline comes from the Sinhala word “turmali”, meaning mixed. Tourmaline occurs in various colours and combinations and no other gemstone variety possesses such extraordinary diversity. In this world of colours, tourmaline can be easily mistaken for other gemstones.

Most tourmalines do not undergo treatment, however for some varieties, the heat treatment is used to improve and enhance the colour.

The definition of tonality is given by the amount of iron, magnesium, copper or lithium in its composition.

The colours are in fact the main characteristic of tourmalines.

Here are some of its best-known varieties:

• Rubellite: the colour varies from medium to deep red, resembling rubies. Rubellite is one of the rarest and most valuable tourmalines. Many gems of the Russian Imperial Crown in the 17th century were believed to be rubies, but they were actually rubellites. The irradiation to obtain the colour is common and gives a stable colour to the gems, and its detection is practically impossible. The heat treatment is also quite common and this usually serves to intensify the colour and make it more uniform.

• Indicolite: from bright blue to blue-green, this variety is considered another rare colour and high quality species are considered exemplary to be collectible.

• Chrome: a tourmaline that is considered different from normal greens, since it is presented in a more intense tone.

• Bicolour: an infinity of frequencies, variations, zones and bands of colours are found.The watermelon tourmaline is one of the most famous, since it is the only one in nature that naturally has two or more colours and shades.

• Canary: tourmaline with a distinctive bright yellow.

• Paraíba: the rarest tourmaline, which has an intense “neon blue” or turquoise colour allied with an impressive luminosity. A mixture of the tones of the sky and the waters that embellish the state that gives them their name.

    Paraíba Tourmaline

    The world of coloured stones changed when, in the late 1980s, copper-rich tourmalines (elbaites) appeared in the market in intense neon blue colours from the region of São José da Batalha, in the state of Paraíba, Brazil. Because of their rarity, they quickly reached prices of thousands of dollars per carat, until then almost only possible in diamonds, emeralds, sapphires and rubies. The commercial expression “tourmaline of Paraíba” has been, since then, associated with tourmalines of superlative value and quality. Tourmalines of similar characteristics were also found in the contiguous state of Rio Grande do Norte, which guaranteed some production of these gems during some years.

    It is nowadays considered that the term “tourmaline of Paraíba” corresponds to a tourmaline (elbaite) of medium to high saturation and intensity of blue and green “neon”, “electric” colours, due mainly to the presence of copper and/or manganese, and coming from various parts of the globe, being that the word Paraíba is allusive to the locality where it was first discovered and not to the geographical origin of the factual samples.

    Gemmological aspects:

    • Variety: Paraíba tourmaline, cat's eye tourmaline, indicolite tourmaline etc.
    • Group: Tourmaline – Elbaite
    • Colour: colourless, blue, green, pink, violet, bicolor, brown, yellow, black
    • Hardness: 7—7.5 (Mohs scale)
    • Specific weight: 2.82—3.32
    • Refractive index: 1.614—1.666
    • Fluorescence: weak or none

    Beryl

    Some beryl varieties are considered gemstones since prehistoric times. The blue beryl (due to chrome and vanadium) is called aquamarine, the green beryl (due to the presence of the chrome element) is called emerald, the rare red beryl is called red emerald, scarlet emerald or bixbyite, the pink beryl (due to manganese and iron) is morganite, the bright yellow and clear beryl is called golden beryl, the colourless beryl is called gochenite and the greenish-yellow (due to manganese, iron and titanium) is called heliodor.

    Most aquamarine crystals come from geological deposits of pegmatite rock, many of which are in Brazil, which makes the country the largest source of aquamarine in the world.

    The world's largest aquamarine gemstone, named Dom Pedro and extracted in Minas Gerais in the 1980s, was donated to the Washington Museum of Natural History and will be part of the permanent exhibition of the institution.

    Gemmological aspects:

    • Variety: aquamarine, cat-eye aquamarine
    • Group: Beryl
    • Chemical composition: Be3Al2Si6O18
    • Colour: blue-greenish to blue-green, usually light tone
    • Hardness: 7.5—8 (Mohs scale)
    • Specific weight: 2.72 (+0.18, -0.05)
    • Refractive index: 1.577—1.583 (± 0.017)
    • Fluorescence: none / inert

    Amethyst

    The amethyst is the most precious stone within the quartz group. Its purple colour was part of the history of the British Imperial Crown and was also one of the favourite stones of the Egyptian royalty.

    The amethyst colour agent is iron, and the variety of shades ranges from purple, pale violet to reddish violet. The darker colours are the most valuable. Green amethysts are produced by heat treatment.

    The most important gemmological deposits are in Brazil, more specifically, in the State of Rio Grande do Sul.

    In addition to the use in jewellery, amethyst quartz is also highly sought after by mineral collectors, both in small samples (“micromounts” and “thumbnails”) and in samples of metric size (amethyst geodes of Rio Grande do Sul – Brazil).

    Gemmological aspects:

    • Variety: amethyst, ametrine (bicolour variety of amethyst with citrine)
    • Group: Quartz
    • Chemical composition: SiO2
    • Colour: from blueish purple to pure purple and reddish purple
    • Hardness: 7 (Mohs scale)
    • Specific weight: 2.66 (+0.03, -0.02)
    • Refractive index: 1.544—1.553
    • Fluorescence: usually inert, may exhibit weak blue fluorescence under light

    Emerald

    Emeralds are the most precious stones of the Beryl group. Its name comes from the Greek smaragdus and the ancient French esmeralde meaning green gemstone.

    Almost all natural emeralds contain inclusions, so they are generally more fragile than other beryls and almost all gemstones are treated with resin to mitigate this fragility. Thus, the most determining factor in the value of the stone is its colour.

    Colombian emeralds are in a shade of bright blueish green, irrespective of their geographical origin. Lighter-coloured emeralds are generally known as Brazilian emeralds.

    Brazil has several deposits, located in Bahia, Goiás and Minas Gerais. In Brazil, also a rare type of emerald is found, known as cat-eye emeralds and more rarely still, emeralds with a six-ray star.

    Gemmological aspects:

    • Variety: trapiche emerald, asterism emerald, cat-eye emerald and emerald
    • Group: Beryl
    • Chemical composition: Be3Al2Si6O18
    • Colour: from light green to very dark to very bluish green
    • Hardness: 7.5—8 (Mohs scale)
    • Specific weight: 2.72 (+0.18, -0.05)
    • Refractive index: 1.577—1.583 (± 0.017)
    • Fluorescence: normally inert, but in emeralds treated with oil, the oil in the fractures may show yellowish green to greenish green or weak to inert fluorescence

    Garnet

    Garnet originates from the Latin word granatus which means grain, and is associated with the rounded form. Reddish-coloured garnets are found more frequently, but it is also possible to find green, yellow, and orange colour garnets and their variations.

    Here are some varieties of the group of garnets:

    Almandine: orange to red, slightly violet red to reddish violet coloured garnets.

    Andradite: yellow, green, brown and black garnets.

    Spessartite: orange-yellow to orange-red garnets.

    Grossularite: colourless (rare), from light to dark yellow to reddish orange, from light to dark green garnets.

    Hessonite: brown to red garnet.

    Malaysian and with colour change: slightly pinkish orange, reddish orange, yellowish orange from light to dark coloured garnets; with colour change – wide variability of colour behaviour between daylight and incandescent lighting, but predominantly blue tints.

    Pyrope: is a red-blood-coloured garnet due to its content of iron and chrome. The pyrope rarely has inclusions, but when present, they are in the form of rounded crystals or have irregular contours. The locations of deposits include South Africa, Argentina, Australia, Brazil, Switzerland and Tanzania.

    Rhodolite: from light purplish red to dark reddish purple colours.

    Tsavorite: green to emerald green garnet.

    Gemmological aspects:

    • Group: Garnet
    • Colour: colourless and all colours
    • Hardness: 7—7.5 (Mohs scale)
    • Specific weight, chemical composition and other specifications: different for each member of the group
    • Fluorescence: usually inert

    Sapphire

    Blue sapphires belong to the group of hematite, a mineral called corundum. Red corundum is known as ruby, and the other colours are sapphires, with blue being the most classic colour of this gemstone.

    As blue is the most popular colour, colour intensity is one of the predominant factors, thus, blue sapphires of richer intensity will always be of the highest value, and very dark or very light-coloured sapphires with lower values.

    Pink, yellow, green, white and multicolored sapphires are often less valued than the blue variety of the same quality and size. However, the pink/orange sapphire, called Padparascha or Padparadja, is considered a good investment.

    They are rare in Brazil, existing in Mato Grosso, Goiás, Santa Catarina and Minas Gerais. And currently the largest producers of sapphires are Sri Lanka and Madagascar.

    Gemmological aspects:

    • Variety: sapphire with colour change, sparry adamantine and asterism sapphire
    • Group: hematite
    • Chemical composition: Al2O3
    • Colour: green, yellow, pink, purple, violet, brown, black, grey, colourless, purplish blue to greenish blue
    • Hardness: 9 (Mohs scale)
    • Specific weight: 4.00 (+0.10, -0.05)
    • Refractive index: 1.762—1.770 (+0.009, -0.005)
    • Fluorescence: different for each colour

    Ruby

    Its name comes from the Latin rubeu, which means “red”. Ruby, like the sapphire, is a variety of the mineral called corundum. This stone is of red colour, because it has the presence of chromium in its composition. Rubies from Burma are the most valuable, but the rubies from Madagascar, Mozambique and Tanzania are now also being well evaluated.

    Gemmological aspects:

    • Variety: ruby
    • Group: Hematite (species: corundum)
    • Chemical composition: Al2O3
    • Colour: from orange red to purplish red, brownish red
    • Hardness: 9 (Mohs scale)
    • Specific weight: 4.00 (± 0.05)
    • Refractive index: 1.762—1.770 (+ 0.009, -0.005)
    • Fluorescence: Burmese – bright red (UVL), moderate red (UVC); from Sri Lanka – bright red-orange (UVL), moderate red-orange (UVC); Thai – light red (UVL), inert (UVC).