Garnet

Garnet Gemstones

• Hardness

  7.0 - 7.5.
  

• Occurrence

  U.S. (Arizona), South Africa, Argentina, Australia, Brazil, Myanmar (Burma), Scotland, Switzerland, Tanzania
  

• Appearance

  Garnet is the name which can be applied to six similar mineral species, namely almandine, pyrope, spessartine, grossular, andradite and uvarovite. To further complicate matters, many garnets are actually a combination of these minerals. Rhodolite garnet for instance, is a combination of almandine and pyrope, and is sometimes referred to as pyrope-almandine garnet. There are also many trade names and other commonly used names which only adds to the confusion, such as Rhodolite, Tsavorite, Hessonite, Malaya, Mozambique, Mandarin, Ant-hill, Leuco, Hydrogrossular, Demantoid, Melanite, Topazolite, Thai. Other names such as "cape ruby" are simply misleading and deceptive. Some garnets also exhibit color change and stars.
  

• Enhancements

  Garnet in general is not enhanced.
  More information on gemstone enhancements.
  
  

  Garnet

  
  Garnets  are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives.
  
  Garnets possess similar physical properties and crystal forms but different chemical compositions. The different species are pyrope, almandine, spessartine, grossular (varieties of which are hessonite or cinnamon-stone and tsavorite), uvarovite and andradite. The garnets make up two solid solution series: pyrope-almandine-spessarite and uvarovite-grossular-andradite.
  
  
  

  Properties

  
  Garnet species are found in many colors including red, orange, yellow, green, blue, purple, brown, black, pink and colorless. The rarest of these is the blue garnet, discovered in the late 1990s in Bekily, Madagascar. It is also found in parts of the United States, Russia, Kenya, Tanzania, and Turkey. It changes color from blue-green in the daylight to purple in incandescent light, as a result of the relatively high amounts of vanadium (about 1 wt.% V2O3). Other varieties of color-changing garnets exist. In daylight, their color ranges from shades of green, beige, brown, gray, and blue, but in incandescent light, they appear a reddish or purplish/pink color. Because of their color-changing quality, this kind of garnet is often mistaken for Alexandrite.
  A sample showing the deep red color garnet can exhibit.
  
  Garnet species' light transmission properties can range from the gemstone-quality transparent specimens to the opaque varieties used for industrial purposes as abrasives. The mineral's luster is categorized as vitreous (glass-like) or resinous (amber-like).
  
  
  

  Crystal structure

  
  Garnets are nesosilicates having the general formula X3Y2(Si O4)3. The X site is usually occupied by divalent cations (Ca2+, Mg2+, Fe2+) and the Y site by trivalent cations (Al3+, Fe3+, Cr3+) in an octahedral/tetrahedral framework with [SiO4]4− occupying the tetrahedra.Garnets are most often found in the dodecahedral crystal habit, but are also commonly found in the trapezohedron habit. (Note: the word "trapezohedron" as used here and in most mineral texts refers to the shape called a Deltoidal icositetrahedron in solid geometry.) They crystallize in the cubic system, having three axes that are all of equal length and perpendicular to each other. Garnets do not show cleavage, so when they fracture under stress, sharp irregular pieces are formed.
  
  
  

  Magnetics used in garnet series identification

  
  For gem identification purposes, a pick-up response to a strong neodymium magnet separates garnet from all other natural transparent gemstones commonly used in the jewelry trade. Magnetic susceptibility measurements in conjunction with refractive index can be used to distinguish garnet species and varieties, and determine the composition of garnets in terms of percentages of end-member species within an individual gem.
  
  
  

  Synthetic garnets

  
  The crystallographic structure of garnets has been expanded from the prototype to include chemicals with the general formula A3B2(C O4)3. Besides silicon, a large number of elements have been put on the C site, including Ge, Ga, Al, V and Fe.
  
  Yttrium aluminium garnet (YAG), Y3Al2(AlO4)3, is used for synthetic gemstones. Due to its fairly high refractive index, YAG was used as a diamond simulant in the 1970s until the methods of producing the more advanced simulant cubic zirconia in commercial quantities were developed. When doped with neodymium (Nd3+), these YAl-garnets may be used as the lasing medium in lasers.
  
  Interesting magnetic properties arise when the appropriate elements are used. In yttrium iron garnet (YIG), Y3Fe2(FeO4)3, the five iron(III) ions occupy two octahedral and three tetrahedral sites, with the yttrium(III) ions coordinated by eight oxygen ions in an irregular cube. The iron ions in the two coordination sites exhibit different spins, resulting in magnetic behaviour. YIG is a ferrimagnetic material having a Curie temperature of 550 K.
  
  Another example is gadolinium gallium garnet, Gd3Ga2(GaO4)3 which is synthesized for use as a substrate for liquid-phase epitaxy of magnetic garnet films for bubble memory and magneto-optical applications.