What are treated gemstones?
Many gemstones are treated to enhance the colour or the clarity of the stone. Common treatments include Heat, Dye/Bleach, Oil, Radiation and Fracture Filling. In some cases (Like Tanzanite), the heat treatment does not affect the value of the stone, however in other cases a treatment can reduce the value significantly. It is recommended to have a stone certified and tested (if possible) for treatments.
Where possible, I have sourced certified coloured gemstones, so I can know the history of those stones - I also prefer to source untreated stones where possible, so my jewellery is truly showcasing the amazing gems that nature provides!
Nearly all tanzanite is heated at low temperatures to remove grey, green and brown tones in the naturally mined stone. Some Tanzanite comes out of the ground blue, normally because the deposits lay in thermal vents and have been heated naturally for millions of years, however even naturally blue stones will be heated to remove any brown undertones.
While heating Tanzanite may result in a more favourable colour, it does not enhance the grade or value of the gemstone. Heat treatment does not affect Tanzanite quality.
My Tanzanites are all incredible quality - I sourced the highest grade in Tanzanite - the very top 1%!
Citrine and Ametrine:
Citrine is a yellow variety of quartz. Natural citrine is rare, in fact most citrine is made by heating amethyst, and partial heating with a strong gradient results in “ametrine” – a stone partly amethyst and partly citrine.
Morganite might be heat-treated to alter the orange-pink colour to a more pure pink. The treatment is routine.
Heating removes yellow tints in aquamarine. Although my beautiful aquamarine and diamond pendant is natural sea-foam coloured which I actually prefer!
Many emeralds are heavily treated. Most emeralds have a wonderful included interior - a mossiness that I’ve heard called le jardin, The inclusions are tolerated because the finest emeralds display a vivid green that is highly coveted. But in many cases the various internal gas bubbles, embedded crystals, veils and cracks make the emerald look cloudy.
The traditional treatment for emerald is fracture-filling with clear or coloured oil. But the oil can dry out and emeralds have to be re-oiled from time to time to keep them looking their best. Because the oiling process requires heat and pressure to force the oil into the gem, it is not a trivial matter to maintain an emerald.
In the 80s, a synthetic resin was used - it has better stability than oil, and a refractive index almost identical to emerald. But many of the emeralds that were treated this way came to market without disclosing their treatments, and dealers and buyers became especially nervous when it was discovered that some emeralds had been treated with coloured resin.
Since then many other polymer fillers have been used to enhance emeralds. Gemological labs are still trying to classify these fillers and understand their stability and long term performance.
The proliferation of enhancements for emeralds means there is a loss of trust in the emerald market and has led to questions about the value of emeralds treated with these new fillers. If I were to invest in emeralds or add them in to my collection, I would only use untreated emeralds.
I’ve always wanted to buy an antique Art Deco style emerald ring for myself, but testing the stones for treatments when already set in a ring is tricky - so for now I shall keep dreaming! (or I’ll design one incorporating a Tsavorite instead)!
Sapphires and Rubies:
These can be heated to high temperatures and held at those temperatures for days or weeks to enhance the colour. Some larger stones can also fracture filled to improve their appearance.
Some pearls can be dyed or bleached to attain more desirable colours.
Topaz gemstones are irradiated to enhance, change or deepen their colour to the most desired blues. It sounds dangerous to wear right? But luckily it isn’t - take a look at this article by the United States Nuclear Regulatory Commission.
Natural pink and blue diamonds are incredibly rare and expensive, sometimes selling for many millions of dollars. However irradiating clear diamonds can change them to blue, green, canary yellow, or even pink. If you see a fancy coloured diamond with a reasonable price, it has most likely been irradiated.