Diamonds and Diamond Simulants

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Remarkable facts:

  • All diamonds are at least 990,000,000 years old

  • Many are 3,200,000,000 years old (3.2 billion years)!!!
    How do we know this?
       Age: from Carbon dating?  NO!   C-dating only works for very young carbon. You need to use other radioactive decay schemes (e.g., uranium-lead) to date inclusions in diamonds. Inclusions used for dating are around 100 microns in diameter (0.1 mm).
  • Diamonds are formed deep within the Earth: between 100 km and 200 km below the surface.

  • Diamonds form under remarkable conditions!
    • The temperatures are about 900 - 1300 C in the part of the Earth's mantle where diamonds form.
    • The pressure is between 45 - 60 kilobars. (kB)
      • 50 kB = 150 km = 90 miles below the surface
      • 60 kB = 200 km = 120 miles below the surface
  • Diamonds are carried to the surface by volcanic eruptions.

  • The volcanic magma conduit is known as a kimberlite pipe or diamond pipe. We find diamonds as inclusions in the (rather ordinary looking) volcanic rock known as kimberlite.

    NOTE: The kimberlite magmas that carry diamonds to the surface are often much younger than the diamonds they transport (the kimberlite magma simply acts as a conveyer belt!).

  • Diamond is made of carbon (C), yet the stable form (polymorph) of carbon at the Earth's surface is graphite.
  • To ensure they are not converted to graphite, diamonds must be transported extremely rapidly to the Earth's surface.

  • It is probable that kimberlite lavas carrying diamonds erupt at between 10 and 30 km/hour (Eggler, 1989). Within the last few kilometers, the eruption velocity probably increases to several hundred km/hr.
  • Diamond is the hardest material.

  • Diamond is the hardest gem on the MOHS harness scale and graphite (also made from carbon atoms) is the softest! Given that both diamond and graphite are made of carbon, this may seem surprising.

    The explanation is found in the fact that in diamond the carbon atoms are linked together into a three-dimensional network whereas in graphite, the carbon atoms are linked into sheets with very little to hold the sheets together (thus the sheets slide past each other easily, making a very soft material).

  • Diamonds are found in many localities, both overseas and in the US.
Basic Data
Famous Diamonds
Carat Weight

Famous diamonds

This is just for fun -- not required information!

How rare are diamonds?

How many grams do you need to mine to get 5 grams of diamonds?

(5g/1000 kg) @ 1000 g/kg = 5 g /1,000,000 g!

But only 20 % are gem quality (80 % of these are sold in a "managed selling environment") and the remainder are used for industrial purposes (this material is known as "bort" or "carbonado" (carbonado is finer)).

This movie (68 k) emphasises that diamonds do not form in the kimberlite magma, but are carried up to the surface by the magma.

Basic Data

  • Hardness = 10
  • Crystal System = cubic
  • This is what crystals look like before they are faceted: note their natural octahedral shape! Uncut diamonds are also found in cubic forms.
  • Diamond has four good cleavages, thus diamonds tend to cleave on impact.
  • Refractive Index = 2.42
  • Dispersion=0.044
  • Specific Gravity = 3.52
  • Value

      The 4 "C" words are used to summarize the value determining factors:
      The required basic information describing what is meant by these terms is provided below.
    • Color is determined by 'grading' visual comparison with 'knowns' or by instrumental means.
      • Consider the amount of yellow color (yellowish color decreases the value of a "colorless" stone). In order of increasing yellow content:
          blueish-white -> white -> silver -> yellow
      • 'Fancy', or strongly colored stones have their own appeal and special value.

      • Colored diamonds may be yellow, green or brown, green or shades of pink.

      •  Larger pink diamonds are quite rare and currently very expensive.

      • Natural blue diamonds contain the element boron (B), and this changes the conductivity of the diamonds. Natural yellow diamonds contain the element nitrogen (N).
    • Clarity is decreased by the presence of blemishes or flaws, scratches, nicks, 'naturals' (the original surface of an uncut stone).
      • There are many systems of nomenclature.
      • Some terms include:
        • perfect
        • flawless
        • imperfect
        • very slightly included
        • very very slightly included
        internally flawless
        very, very slightly included
        very slightly included
        slightly included
      • other descriptions:
        • "Perfect," "internally flawless," and "flawless" are not synonymous. "Flawless" is reserved for diamonds having no visible inclusions under 10x magnificantion and having no external blemishes of any kind.
        • Clarity grades refer to what is visible at 10x magnification. With sufficient magnification, inclusions will be revealed in any diamond. "Perfection" is relative.
        • "pique" (used below and in older literature) is an old trade term and has been supplanted by the GIA-developed international standard, from IF to I3.
        • "first pique" inclusions readily recognizable at 10x mag., not significantly diminishing brilliance
        • "second pique" larger inclusions, can be seen with naked eye
        • "third pique" many large inclusions, diminishing brilliance
      • Examples of clarity-reducing inclusions:

    • Cut:  Facets are placed so as to maximize the brilliance and fire of a stone.
      • Remember that in the first lecture we talked about how the proportions of a faceted gemstone are determined based on the refractive index?
      • Review the basic concepts:
        • Refraction is dependent upon the wavelength.
        • Refractive Index (RI) is proportional to wavelength; red RI < violet RI (dispersion is due to the different amounts different wavelength are bent).
        • Fire,which is seen as rainbows and glints of color, is due to dispersion (a consequence of the placement of faces on the crown to take advantage of the prism effect).
      • The brilliant cut (modern round brilliant cut or Tolkowsky cut) is a typical cut chosen for diamonds. Tolkowsky determined the optimal proportions are such that the table width is 53% of the diameter of the cut stone. Appraisers will penalize diamonds with tables above 64%. Significant deviations, up to table widths of more than 70% are not uncommon.
      • There are many alternative diamond cuts.
      • A poorly cut stone is characterized by poorly chosen proportions (poor optimization of brilliance and fire or, worse still, leakage of light from the pavillion). Misplaced facets, extra facets, and problems at facet junctions are also characteristics that reduce the quality of "cut".
      • Ranking: VERY GOOD ... GOOD .... MEDIUM ... POOR
    Review the light path in a correctly cut gem!

    • Carat Weight
      • Recall: 1 carat = 0.2 g, thus 5 carats=1g
      • For example, compare the size of a one point diamond to that of a 0.67 carat diamond.
    Just FYI: This site explains the GIA grading report used for diamonds, including information on desirable characteristics.

    Other issues: Treatment, simulants, synthetics


    • filling of cracks

    • Surface cracks and cleavages reaching the surface are often filled with a glass-like material.

      Identification: when examined with an optical microscope, filled stones will show:

      • greasy appearance
      • flash effects
      • bubbles
      Problem: Filling does not always resist polishing and cleaning
    • drilling of inclusions
    • Drilling inclusions involves using a laser to drillin into the inclusion. Solutions can be poured into the resulting "hair-width" diameter hole to bleach colored inclusions. This is comparaed to getting a filling in your tooth.
    • irradiation

    • Irradiation is used to change the color of the diamond. A common color produced by irradiation is green.

      Early attempts: beginning of 20th Century: diamonds exposed to radium - the problem was that the diamonds remained radioactive! However, modern irradiation treatments do not produce radioctive stones.

      Irradiation involves the use of devices such as:

      • linear accelerators
      • gamma ray facilities
      • nuclear reactors
      Detection of irradiation treatment:

      Electron irradiation only changes the surface of the stone. Thus, it produces a concentration of color where the gemstone is thin. For example, electron irradiation produces a color concentration at the culet or keel line of the faceted gem.

    Above, a diamond with a surface crack.

    Below, examples of irradiated diamonds.

    Simulants - simulate the appearance of diamond

    The distinction between a synthetic diamond (man-made diamond consisting of carbon atoms arranged in the typical diamond structure) and a diamond simulant (not a carbon compound with the diamond structure) is very important!

    In order of increasing R.I., the most common simulants are:

    • YAG = yttrium aluminum garnet
    • GGG = gadolinium gallium garnet
    • CZ = cubic zirconia
    • Strontium titanate
    • diamond.
    This mnenonic can be used to memorize the common diamond simulants in the above order:

    You go crazy staring at diamonds.

    Again: Simulants (look alikes) differ from synthetics (synthesized by humans!)!

    Another diamond simulant, synthetic moissanite (Silicon carbide or carborundum) was introduced to the jewelry market in 1998; manufactured by C3 Inc. and Cree Research. It has become the gold standard for diamond simulants in the last few years.

    Source: Jewelers of America
    Crystal Structure hexagonal
    HARDNESS 9.5
    R.I. 2.65-2.69
    Specific gravity 3.17-3.20

     Simulants are distinguished from diamonds using measurement or observation of various properties, such as:

      • R.I.
      • "Read through effect"
      • Dispersion
      • Hardness
      • Specific Gravity
      • Reflection pattern
      • Shadow patterns
    Note: not all diamond simulants have been around for the same length of time!

    Synthesis (Details on gem synthesis)

    Synthetic diamonds are often yellowish in color (rarely used for gem purposes, more commonly used as diamond grit for industrial purposes. Modern synthesis of thin film diamond has other industrial applications).

    A 5 mm diamond (0.5 carat) takes over a week to grow. Synthesis requires:

    Synthetic diamonds can sometimes be distinguished from natural diamonds by the presence of flux inclusions (Ni, Al or Fe).

    Lots of diamond images!

    Previous Lecture:  How are gems identified?
    Next Lecture:  Color in Minerals

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