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16.50: Opal

  • Page ID
    4943
  •  

    Opal
    Chemical composition SiO2+.H2O
    Crystal system Amorphous
    Hardness 5- 6 1/2
    Refractive index 1.45 (+.020;-0.080)
    Specific gravity 2.15 (+0.07;-0.90)
    Lustre Vitreous
    Phenomenom Play of Color

    Diagnostics

     

    Description

    Opal is a mineral species with an amorphous structure. Its composition is silicon dioxide with a variable amount of water. The amount of water (H2O) is usually between 2-10% by weight, although contents as high as 20% have been recorded. When opals undergo dehydration as a result of exposure and time they have a tendency to become crazed, showing minute cracks on the polished surfaces.

    In 2000, The Australian Gemstone Industry Council established nomenclature and classification standards for all types and origins of opals, This classification has been adopted internationally. 
    Opals can be broken down into 2 basic categories:

    • Precious Opal: any opal displaying play-of-color. This phenomenon is caused by the diffraction of white light thru a microscopic, orderly arrangement of silica spheres. This category includes white, black and boulder opal. Precious opal can be further distinguished by types:

     

     

     

     

     

     

    File:Whiteopal.JPG

    Figure \(\PageIndex{1}\): Mintabe Opal Rub
    Photo courtesy of Kevin Schwebel, Handmade Enterprises

     

    Type 1: A single, solid piece of precious opal, having a uniform appearance and composition. This is the type of opal most commonly used for jewelry
    Type 2: Precious opal that is attached to its host rock (a non-opal) in the form of a layer or seam. Boulder opal is an example of this. The opal is attached to a brown, iron-stained sandstone. 
    Type 3: Matrix opal occurs when precious opal fills cracks and openings in the host rock. The opal forms in pre-existing clay or sandstone. This material is frequently dyed.
    Doublets & Triplets: These assembled stones are not considered natural opals, although they do contain a layer of natural opal.

    • Common Opal or Potch: These are varieties of opal that do not show a play-of-color. Although they share the same chemical composition as precious opal, the silica spheres they contain are randomly arranged. 

    File:YelOpal.jpg

    Figure \(\PageIndex{2}\): Yellow Common Opal
    Photo courtesy of Rick Martin, Art Cut Gems

     

    Color

     

    Opal tones.gif 

    Figure \(\PageIndex{3}\)

    Judge the body color of an opal "face up". 
    Example: 
    If an opal has very dark potch on the back, giving it the appearance of N5, it should be graded as N5.

     

    Transparency

     

    File:Opalmex.jpg

    Figure \(\PageIndex{4}\): Precious Mexican Crystal Opal Photo courtesy of Rick Martin, Art Cut Gems

     

    Opals can vary in degrees of transparency from transparent to opaque. When an opal is transparent or semi-transparent it is referred to as "crystal". This is true regardless of the body tone. "Crystal" refers to the glass-like appearance of the gem, NOT a crystalline structure.

     

    UV Reactions

     

    Black opal is usually inert, but light varieties of opal, both common and precious may fluoresce in both LW and SW ultraviolet light. Some natural opals phosphoresce green after exposure to LW ultraviolet light. Synthetic opals do not.

     

    Chelsea Colour Filter

     

    No diagnostic reaction.

     

    Treatments

    Figure \(\PageIndex{5}\): Opal treated with carbonized sugar and acid

     

    • Treatment with aniline dye, silver nitrate, or sugar carbonized with acid.
    • Impregnation with oil, wax, or plastic. 
    • Smoke impregnation. 
    • Impregnation with black plastic.
    • Backing with foil, black paint, or laquer.

     

    Phenomena

    Play-of-Color.

     

    Synthetics

    The firms of Gilson in France and Chatham in the United States currently create synthetic opals. They are produced with white and black body colors. Chatham also markets a "crystal" type which exhibits a colorless body with play of color.
    This laboratory-created material was first marketed by Pierre Gilson, Sr. in 1974. It involves a 3 stage process involving the purification of chemicals involving fractional distillation. This is characteristically an elaborate process involving the consolidation of these refined chemicals under water pressure. The result is the creation of microscopic cristobalite silica spheres (SiO2) of uniform size. They are allowed to settle in the containment vessel for over a year in a hydrous solution of controlled pressure and acidity. Then, a hydrostatic press is utilized to consolidate the microspheres from a liquid phase into a solid phase, resulting in synthetic opal rough which can be cut into cabochons.

     

    Imitations

    Slocum Stone: Glass Imitation (separation: magnification, RI, SG)
    Japanese Plastic with Play of Color (separation: SG, Hardness: pressure with a pin will make a slight indentation)

     

    G&G Articles on Opal 1934-1980

    The GIA has published all the G&G's from 1934 until 1980 online. The organization of the list by subject was done by Joseph Gill.

     

     

    Sources

    • Gems Their Sources, Descriptions and Identification 4th Edition (1990) - Robert Webster/ B.W. Anderson
    • Secrets of the Gem Trade, by Richard W. Wise
    • A Student's Guide to Spectroscopy (2003) - Colin H. Winter
    • Gem Identification Made Easy 3rd edition (2006) - A.C. Bonanno/ Antoinette Matlins
    • GIA Gem Reference Guide for the GIA Colored Stone & Gem Identification Courses