Maybe think twice
BY Kaylan Khourie, FGA
Sunstone falls part of the Feldspar mineral group. It is known for exhibiting what is called “Aventurescence. Aventurescence is a type of iridescence (a play-of-colour) that is caused by the reflection of small, thin and platy inclusions – copper, goethite and/or hematite in the case of Sunstone – that are spread in a parallel orientation through the gem.
This causes interference of light between the layers of platelets which creates the glittery sheen associated with Sunstone.
The larger and more abundant the inclusions are, the more “Aventurescent” the stone will be and the deeper the golden colour will appear.
There are also transparent Sunstones from Oregon in the USA. They are often green and/or red in colour, with small copper inclusions (often in “streams”) creating a “Schiller” effect.
From left to right –
Orthoclase Sunstone, Oregon Sunstone.
Sunstone falls into three species of the Feldspar group depending on its chemistry: Orthoclase, Oligoclase and Labradorite (this is also into where the Oregon material falls). Orthoclase falls under the “Alkali Feldspar” (also known as “K-Feldspar”) category whereas Oligoclase and Labradorite fall under the “Plagioclase Feldspar” category.
Below is a table of each species of Sunstone and some of their properties:
However, the gems being marketed as “Sunstone” are almost always pieces of man-made glass containing an abundance of tiny copper inclusions. The correct name for this is “Aventurine Glass” – however the misnomer “Goldstone” is more popular for marketing purposes. Ironic, given that it is coloured by copper, not gold.
Uncut and tumbled pieces of Goldstone.
Goldstone vs Sunstone
Imitation gems are usually manufactured to have properties (physical and optical) that are close to the gem that they are imitating – either to create an appearance as close to the imitated gem as possible or as an attempt to make its real identity difficult for the gemmologist to discover.
This is why Goldstone has a refractive index and/or specific gravity that overlap with the different Sunstone species:
Abundance of tiny triangular and hexagonal platelets of copper.
The defining factor is that glass is amorphous (which means that it has no crystal structure) and therefore “singly refractive” – only one ray of light passes through the gem. Whereas Feldspar is part of either the Monoclinic (Labradorite and Oligoclase) or Triclinic (Orthoclase) crystal system – which means they have asymmetrical crystal axes lengths and angles. So their crystals grow at different rates and angles and refract the light that passes into them unevenly. This is the reason why they are “doubly refractive” – the gem splits the light into two rays.
The visual appearance and microscopic observations are also very useful in identifying Goldstone because it is coloured by an abundance of tiny triangular and hexagonal platelets of copper.
Inclusions in Sunstone
Similarly to Goldstone, Sunstone is typically included with copper platelets – however they additionally contain long hematite and/or goethite platelets. As mentioned above, this is the reason for the Aventurescence.
Here are some images of inclusions and features related to Sunstone:
From left to right –
Layered platelets of goethite and hematite. Manganese stains, which although uncommon, may occur.
Full disclosure should always be given!
Imitations of natural gems will always be readily available because it enables the consumer to buy something that looks similar to its natural counterpart. This is not a problem since the simulant, for the most part, has a very similar appearance to that of the natural gem – and it comes at a fraction of the cost.
What is important is that full disclosure about a gem should always be given so that the consumer, who may not always be familiar with trade terminology, understands exactly what they are buying and what its fair market value is. For example, one should never buy a cubic zirconia under the impression that it is a diamond, at a diamond’s price.
Therefore, it is always wise to insist on a gemmological certificate of some kind from a reliable laboratory when purchasing a gemstone, especially one of significant value. This industry can be somewhat untrustworthy at times, so being sure of a gem’s true identity can certainly save you a lot of money. So be weary when being offered a desirable and usually expensive gem, at a relatively “cut-price” deal.