Emeralds differ from corundum mineral – known for ruby and sapphire – which has the same refractive index (RI) and specific gravity (SG) regardless of being natural or synthetic. The RI and SG for emerald generally vary between synthetic flux, synthetic hydrothermal and natural gemstones so these constants can point a gemmologist in the right direction when observing inclusions.
With set gemstones it may be possible to obtain the RI but not the SG and, as the RI for the flux-grown emerald is lower than for hydrothermal and natural gemstones, this is helpful information when viewing the inclusions.
The RI for hydrothermal emeralds is higher than flux and generally lower than the natural gemstones, although they may overlap. Again, this is a guideline for recognising the types of inclusions visible.
Flux-grown synthetics may have coarse or fine flux inclusions, often opaque (Figure 1). They may be wispy and are not liquid. In contrast, hydrothermal inclusions may consist of liquid feathers, sometimes in a spiral shape. They may present as roiled to angular growth zoning, appearing as ‘chevron’ shapes, or the growth zoning may appear as a slightly wavy, undulating pattern (Figure 2). If a synthetic hydrothermal emerald has been grown in a gold-lined crucible, gold inclusions may be found. To date, no gold inclusions have been found in natural emeralds.
Natural emeralds from different locations present different types of inclusions. While some may contain two or three-phase inclusions, the spiky three-phase inclusions were thought to be diagnostic for Colombian emeralds but these have since been found in emeralds from other countries, including Australia. Figure 3 shows an example of a spiky three-phase inclusion from Torrington in New South Wales.
With appropriate lighting, brightly-coloured liquid films parallel to the basal pinacoid are found in many natural beryls, including emerald. Crystals such as biotite mica, phlogopite, calcite, pyrite, actinolite and many others may be also discovered as inclusions.
Emeralds are also subjected to various treatments, such as oiling, fracture filling and dyeing, in order to lessen the appearance of cracks; surface cracks and cavities may be in-filled with plastic, epoxy, or wax, again to reduce the visibility of surface area flaws.
They may be coated with such products as lacquer, paint or stain to enhance their colour.
In summary, natural emeralds from different locations generally provide evidence of different inclusions and synthetic emeralds made by different processes exhibit different inclusions from one another as well as from natural gemstones.
Retailers need to be aware that some treatments greatly affect the price of a gemstone. The Australian Competition and Consumer Commission (ACCC) states that businesses in Australia must be able to guarantee what they sell.
If gemstones are treated, this must be disclosed to the purchaser and written on their receipt.
So if a natural emerald has been dyed to enhance colour, for example, this must be written on the receipt whether the seller is a supplier or a retailer.