Gemstones , partly as gems called, are mostly minerals , rocks or glass melts , as well as materials of organic origin, such as amber , coal pitch or relatively small and shapely fossils that generally as well be perceived and as jewelry used. According to the definition of the international trade organization CIBJO , pearls , mother-of-pearl and corals also count among the gemstones.
The study of precious stones and semi-precious stones is called gemology .
Stones have also been used as jewelry since the Paleolithic Age . In ancient times , precious stones were processed into jewelry in addition to gold , silver and other materials. Ruby , emerald , sapphire and beryl were used early on, also mentioned in the Bible . Amber was also used as a gemstone.
Since gemstones were usually of considerable value, they were often forged. The color of some minerals, such as agate , has been changed by burning or coloring. These and some other traditional "enhancements" do not need to be declared. However, if there are color changes due to exposure to electromagnetic waves, this must always be stated.
In ancient and medieval times were jewels only more or less round ground . The facet cut only appeared in the early modern era . The diamond , too, has only become a gemstone in modern times, while in antiquity it was used by craftsmen due to its hardness, for example for carving gems .
Minerals of the appropriate quality are often used as precious stones or gemstones. Depending on the type of mineral (e.g. diamond , corundum , malachite ), different criteria are used to determine the quality. The location can make a difference in fine details of the individual characteristics, which in turn reveal the origin of the stone to a specialist.
Often the transparency, purity, rarity and color determine the use and the value. Some minerals have inclusions that reduce the value of the stone, but can also increase it. In the case of one of the most valuable gemstones, the diamond, four properties (4 C) are used, the cut, the weight in carat ( carat ), the color (color) and the clarity (clarity), which differ for a long time let humans influence only the first. In the meantime, however, inclusions that affect the purity of diamonds can be artificially reduced.
Gemstones are partially treated with heat or radioactivity in order to improve or change their optical properties. For example, the color of some amethysts changes from purple to yellow after heat treatment. The treated mineral is then marketed as "citrine". In Germany, these artificially treated minerals must be labeled accordingly, for example in the event of color changes.
Jewelery-quality minerals are also produced synthetically, for example quartz with its amethyst or corundum variety . The quality of synthetic diamonds has been greatly improved in recent years, so that they are also used as gemstones.
Gemstones are gemstones that meet the following three criteria:
Well-known types of gemstones include ruby , sapphire , emerald and topaz . A diamond is a special crystalline form of elemental carbon. According to the above definition, today it also belongs to the precious stones, while in the Middle Ages it had no special value as a gem stone and mostly only the colored stones were called precious stones.
Today, precious stones are usually cut into shapes that increase light reflection and enhance the shine through the quality of the polish, but also to give the mineral a shape suitable for further processing in jewelry. Only brilliant-cut diamonds are referred to as brilliant-cut diamonds, other brilliant-cut gemstones must be supplemented with the gemstone name.
The term semi-precious stones is out of date and used to refer to gemstones that are characterized by their beauty, but in contrast to the "real" gemstones occur much more frequently in nature, and were usually less hard and therefore less valuable.
In mineralogy and gemology, the term “semi-precious stone” is generally no longer used; instead, only precious stones or gemstones are used. On the one hand, the classification was arbitrary, since adjectives such as “real” and “semi-precious” cannot be properly defined for gemstones. On the other hand, the term semi-precious stone indicates a certain inferiority, which is actually not present.
In addition to the classification features already mentioned above, such as light transmission, purity and color , there are also the following criteria, which are based on the criteria for mineral determination:
The criteria used include the chemical composition, for example diamonds consist of carbon , rubies of chrome-colored aluminum oxide (Al 2 O 3 ). Precious and semi-precious stones are also differentiated according to their crystal system , the type of crystal lattice, which can be cubic, trigonal or monoclinic, for example. The so-called habitus , the form in which the stone can be found in nature, is another classification criterion.
Gem and gemstone types are often further divided into different varieties, which mainly depend on the color. For example, red corundum is traded as ruby , a special red-orange as padparadscha . The remaining colors are called sapphire , with the blue sapphire being the most valuable. Diamonds can also come in different color tones, which are then known as "fancy diamond". The varieties of beryl are found as emerald (green), aquamarine (blue), Red Beryl (also outdated Bixbite , red), Goshenite (colorless), beryl (lemon yellow to golden yellow) or Heliodor (light green) or morganite (also Rosaberyll ) . Heliodor is, however, partially rejected as an independent beryl variety and counted among the gold beryls.
Physical differences manifest themselves in the refractive index , the dispersion , the specific density , the hardness , cleavage , brittleness and the gloss . Different gemstones such as tourmalines can show different colors or be birefringent due to pleochroism in different directions . Their absorption spectrum is also characteristic .
After all, its size also plays an important role in the value of a precious stone or gemstone.
The usability and the value of a gemstone / gemstone is based on criteria that are very dependent on the type of mineral. For ores such as hematite and pyrite there are no other criteria than gloss and, under certain circumstances, shape. With garnets, quartz and other minerals, transparency, purity and color also play a role.
Manipulations and imitations
Many minerals or rocks that are used as gemstones are manipulated in various ways in order to improve their properties (color, shine, durability) and thus make them more desirable or to imitate other rare and valuable gemstones.
Oils / fats
One of the oldest methods of repairing stones is to oil them to cover up cracks. The stone appears more transparent and the colors brighter and more intense (compare wet and dry river pebbles ). The oils used range from animal oils (whale rat, tallow) to vegetable fats ( vegetable oil , olive or sunflower oil ) to synthetic oils and even baby oil ( Vaseline ).
Oiled stones easily “sweat out” the oil when heated, and it dries up over time. Both lead to staining and loss of gloss. Oiled rough stones and minerals can possibly become completely unusable due to the formation of an ugly, non-washable coating. Only when using colored oils, the addition is dyed duty. However, among collectors it is considered incorrect to offer such pieces without labeling, as the (value-determining) visual impression of the pieces is significantly changed.
Waxing / paraffining
Instead of using oil, you can also use wax or paraffin to cover cracks and increase shine and color. Paraffining is a little more durable and is mainly used for opaque gemstones and tumbled stones . However, the wax will also wear away over time through use or strong heat radiation. The waxing, if it is colorless, does not have to be specified in the trade. This method is also not considered by collectors.
Soft, porous or coarse-grained gemstones are treated with a resin or synthetic resin coating to protect them from damage from scratches and chemicals (sweat, soap). However, the color can also be changed here by using colored resins. Stabilized gemstones must be treated with the addition .
Reconstructions are especially common with opaque stones, but this method is also popular with amber. Here pulverized material or small fragments are either fused together ( amber ), sintered ( hematite ) or glued with a suitable binding agent ( malachite , turquoise ). Reconstructions of amber can be called "real amber", but hematite is renamed hematin. All other gemstones that do not have a separate trade name must be designated as "reconstructed".
Gemstones with an undesirable or too pale color are recolored using various, mostly superficial, methods in order to enhance them. All colored gemstones must also be designated as such.
- Colored oils, waxes or plastics
- are common means of recoloring gemstones. However, only porous stones can be colored through or at least deep colored. With all others, the colorant is on the surface or, in the case of cracked stones, at most a few millimeters deep. For example, agates are heated in a color solution for a long time, and carnelian is then burned in order to achieve the final color nuance and to fix the colorant. Superficially colored stones rub off over time, especially with frequent body contact.
- is a very durable and difficult to recognize possibility of dyeing, which also creates an iridescent effect. The treated mineral ( rock crystal , topaz ) is vaporized with metal , mostly gold .
- Soaking in sugar solution
- and subsequent dehydration (dehydration) is mainly used for agate and black opal in order to imitate the rare onyx . However, the black color of black opal is naturally only a few millimeters thick in order not to destroy the water-containing mineral when dehydrated.
- Diffusion treatment
- Here, under special pressure and heat conditions, coloring atoms are introduced into the outer layers of the gemstone.
Firing means that the raw gemstones are heated to several hundred degrees Celsius in order to change their color and transparency. Coloring, metallic inclusions are oxidized, crystallization defects and thus cloudiness are dissolved. Depending on the temperature and burning time, different gemstones can have different shades of color. When amethyst is converted into citrine, for example, it takes on a light yellow color at a temperature of around 470 ° C, but a dark yellow to red-brown color between 550 ° C and 560 ° C. Smoky quartz can sometimes be converted at 300 to 400 ° C. Firing changes the stones permanently, but is difficult to detect.
Particularly lucrative is the firing of certain inexpensive, milk-white sapphires, so-called geuda, to a cornflower blue color. An increase in value of 10 to 100 times is possible. Even stones that have already been faceted can be fired in this way if they have few inclusions.
Burned stones do not have to be indicated as such. However, to avoid being misled, it is not permissible to use fantasy names for artificially treated stones.
Irradiation with X-rays or radiation from radioactive substances (gamma, neutron or, more rarely, alpha rays) also serves to change the color, which can be very strong, but in contrast to burning, is not always permanent. In addition, radionuclides are formed in the stone when irradiated with neutrons, which may make the gemstone radioactive. You must therefore be in quarantine until the radiation subsides , which can sometimes take a few years. All gemstones modified in this way must have the addition treated or irradiated .
Some minerals can be produced artificially ( synthetically ) from the corresponding basic elements , for example using the Verneuil process . To produce certain minerals, however, additional heat and pressure are required. The diamond is the best example of this, but many other minerals are now also synthesized in very good quality and show only minor differences to their natural models. In addition to diamonds, syntheses of rubies and sapphires, emeralds, various quartz and opals are particularly widespread. All syntheses must be labeled as such.
Nowadays, toy stores offer special chemical kits which, however, only produce crystals with a gem-like appearance (mostly through recrystallization of potassium alum or similar, harmless salts and any color additives from saturated aqueous solution).
Syntheses are often used for costume jewelry because, in contrast to their naturally created models, they are usually cheaper to manufacture. In this way, inexpensive pieces of jewelery can be produced whose artificially produced syntheses can hardly be distinguished from real gemstones.
Since many minerals look very similar, especially in color, rare and therefore expensive minerals are often imitated by more frequent and therefore cheaper minerals. A common imitation is synthetic spinel, which can be produced in many colors. It is even easier to create imitations using glass or ceramics . In order to distinguish real gemstones from forgeries , their physical properties have to be analyzed.
Doublet / triplet
A special case of imitation is the doublet or triplet, which consists of composite layers of real gemstone and glass, syntheses, quartz or other solids. With this method, you can make many stones from a small amount of basic material. It is z. B. Used real opal, but it is a massive manipulation of the stone. Triplets and doubles must be declared.
Duplicates are a thin layer of the real gemstone that is glued to a base made of obsidian , various iron stones, potch (opaque opal without play of colors) or plastic . This protects the sensitive stones from body and thus sweat contact, among other things. In the case of triplets, there are correspondingly two covering layers, the base protects against body contact, the top layer against scratches and drying out and is therefore particularly often used for opals. In order to be able to recognize doublets or triplets, the stone usually has to be removed from the setting.
In order to find out forgeries, manipulations or imitations, the density or, in the case of translucent minerals, the refractive index of the gemstones to be examined can be used. A refractometer is used to determine the refractive index . Another method is spectroscopic examinations with which the spectral distribution of the characteristic absorption lines in the absorption spectrum can be analyzed.
A simple method for roughly determining the refractive index is also the so-called immersion method , in which the gemstones to be tested are immersed in liquids with a known refractive index. These make the contours of the immersed object disappear if the refractive index matches.
Certain gemstones can also be identified with the help of fluorescence . The two ultraviolet wavelength ranges between 200 and 280 nanometers and 315 and 400 nanometers are primarily used for light excitation . The stones then glow in characteristic colors in visible light .
Rare gemstones and collector's stones
Among the rare and lesser-known gems or stones collectors include Grandidierit , Painite , Jeremejevit , Serendibite , taaffeite (Magnesiotaaffeit 2N'2S) Musgravit (actually Magnesiotaaffeit) Poudretteit , Chambersite , Red beryl ( Bixbite ) and several others.
Medicine and esotericism
Many precious stones and gemstones were also used in medicine ( lithotherapy ) from antiquity to the early modern period , e.g. B. by Marbod von Rennes and Hildegard von Bingen as well as Šams ad-Dīn Moḥammad al-Akfānī († 1348) described, pharmaceutically processed and medically used and are still used today in esotericism as a healing stone or are supposed to protect against bad influences in amulets . Books in which gemstones and minerals and their use are described, such as Aristotle or al-Biruni , are called stone books or lapidaries . One of the oldest German-language stone books is the so-called Prüller stone book , a small lapidary from the 12th century that has been handed down together with the Prüller herb book .
- Bernhard Bruder: Beautified stones . Neue Erde, Saarbrücken 2005, ISBN 3-89060-079-4 .
- Ulrich Engelen: The precious stones in German poetry of the 12th and 13th centuries. Munich 1978 (= Münstersche Medieval Writings. Volume 27).
- Gerda Frieß: Gemstones in the Middle Ages. Change and continuity in their meaning through twelve centuries (in superstition, medicine, theology and goldsmithing). Hildesheim 1980.
- Urban T. Holmes : Mediaeval gemstones , in: Speculum 9 (1934) 195-204.
- Florian Neukirchen: Gemstones: Brilliant witnesses for the exploration of the earth . Springer Spectrum, Heidelberg 2012. ISBN 978-3-8274-2921-6 .
- Adalberto Pazzini: Le pietre preziose nella storia della medicina e nella legenda. Rome 1939.
- Walter Schumann: Precious stones and gemstones . 6th edition, BLV Verlags GmbH, Munich 1976/1989, ISBN 3-405-12488-3 .
- BeyArs.com - Gemstone etiquette by Leopold Rössler (alphabetical overview of most gemstones including their most important properties and treatment methods)
- Institute for Gemstone Examination (EPI) - Name search: trade names and what they mean
- realgems.org - List of gemstones, gemstones, collector stones and minerals
- Heinrich Quiring : The precious stones in the official shield of the Jewish high priest and the origin of their names. In: Sudhoffs Archiv 38, 1954, pp. 193-213.
- Ruhr-Uni-Bochum, Project Diamond: Magic and History of a Gift from Nature , Brochure Rubin 1/03 (PDF; 581 kB).
- The great art dictionary by PW Hartmann - hardness
- See Jan Hirschbiegel: Étrennes , p. 154, footnote.
- Cf. Alois Haas, Ludwig Hödl, Horst Schneider: Diamant: Zauber und Geschichte einer Wunder der Natur , p. 78.
- Walter Schumann: Precious stones and gemstones. All species and varieties in the world. 1600 unique pieces . 13th revised and expanded edition. BLV Verlags-GmbH., Munich et al. 2002, ISBN 3-405-16332-3 , p. 10 .
- Walter Schumann: Precious stones and gemstones. All species and varieties in the world. 1600 unique pieces . 13th revised and expanded edition. BLV Verlags-GmbH., Munich et al. 2002, ISBN 3-405-16332-3 , p. 112 .
- Bernhard brother embellished stones . Neue Erde, Saarbrücken 2005, ISBN 3-89060-079-4 , p. 24 .
- J. Liebertz: Synthetic gemstones. In: Natural Sciences. 65, 1978, p. 501, doi : 10.1007 / BF00439789 .
- Walter Schumann: Precious stones and gemstones. All kinds and varieties. 1900 unique pieces . 16th revised edition. BLV Verlag, Munich 2014, ISBN 978-3-8354-1171-5 , pp. 220, 276 .
- Peter Blinn's Curious Notions - ten of the world's rarest and most exclusive gemstones species
- Mindat - Painite
- Mindat - Red Beryl
- John Marion Riddle: Marbode of Rennes' (1035-1123) "De lapidibus". Stuttgart 1977 (= Sudhoffs Archive , Supplement 20).
- Raimund Struck: Hildegardis De lapidibus ex libro simplicis medicinae: Critical edition comparing other lapidaries. Medical dissertation Marburg 1985.
- Friedrun R. Hau: al-Akfānī, Šamsaddīn Muḥammad. In: Werner E. Gerabek , Bernhard D. Haage, Gundolf Keil , Wolfgang Wegner (eds.): Enzyklopädie Medizingeschichte. De Gruyter, Berlin / New York 2005, ISBN 3-11-015714-4 , p. 24.
- Willem Frans Daems : Gemstones in Medicine. In: The Three. Journal for Science, Art and Social Life 51, 1981, pp. 504-518.
- Hermann Fühner : Lithotherapy. Historical studies of the medicinal uses of the gemstones. Ulm 1902; Reprint 1956.
- Manfred Ullmann : Gemstones as Antidota: A chapter from the poison book of ibn al-Mubārak. In: Janus. Volume 61, 1974, pp. 73-89.
- Gundolf Keil : 'Prüller Steinbuch' (Munich, clm 536, 82 v -83 v ; cgm 5248 [11, Frgm.]). (In the same manuscript fascicle as the 'Prüller herb book' handed down) In: Werner E. Gerabek u. a. (Ed.): Encyclopedia of medical history. 2005, p. 1188.
- Gundolf Keil , Prüler Steinbuch '. In: Author's Lexicon . 2nd ed., Volume 7, Col. 875 f.
- Gundolf Keil: 'Prüller Steinbuch'. In: Werner E. Gerabek et al. (Ed.): Enzyklopädie Medizingeschichte. 2005, p. 1188.