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Detecting Gem Treatments

Gemstones have been enhanced by a variety of methods for centuries. In fact the earliest writings on gem treatments can be found in Pliny the Elder's Natural History, written circa 77-79 AD.

Gemstone enhancements cover a whole range of techniques designed to improve the color and/or clarity of a gemstone. They range from simple heat treatment and oiling, to diffusion with various chemicals, dyeing, bleaching, coating, irradiation and fracture-filling.

It is essential in the gem trade that any gem treatments be disclosed to the buyer, since these enhancements may affect the value of the gemstone and may entail special care for the stone.

Heated Apatite from Madagascar
Heated Apatite from Madagascar
Heated Tanzanite from Tanzania
Heated Tanzanite from Tanzania
Unheated Sapphire from Tanzania
Unheated Tanzanian Sapphire

Like all ethical gem dealers, we are meticulous in disclosing gem treatments to our customers. When we buy a new batch of gems, whether in rough form or cut and polished, we always acquire as much information as we can about where the material was mined and whether any treatments have been applied. Since the information we collect is not always reliable, we also send samples to leading gemological laboratories for analysis. Only then will we offer the gems for sale to our customers.

However, there are limitations to gemological testing, even with sophisticated new equipment such as Infrared and X-Ray Fluorescence Spectrometers. Some gem treatments are currently undetectable by even the most esteemed gem labs.

A treatment will be undetectable if it leaves no measurable trace in the gemstone. Consider for example the heat treatment of sapphire. Sapphire is heated at very high temperatures; usually between 1650 and 1850 degrees Celsius. These high temperatures cause the rutile in the sapphire to dissolve, which increases both the clarity of the stone and the color saturation (through the release of titanium that is absorbed into the sapphire lattice). Skilled gemologists can determine that a sapphire has been heated by observing internal structures under a microscope.

Untreated Purple Tanzanian Sapphire
Untreated Purple Tanzanian Sapphire

However, heat treatment at lower temperatures will not leave any traces that can be observed by gemologists. So even sophisticated gemological labs are unable to detect low temperature heat treatment for a number of gem types, including apatite, beryl, chalcedony, fluorite, quartz, spodumene, tanzanite, topaz, tourmaline and zircon. Nonetheless, if we have good reason to believe that a gem has been heated, we will label it as such even if the lab cannot tell us whether it has been heated for certain. Thus you will find that all of our blue apatite is described as heat treated.

This lack of information can be disconcerting for some customers, but experienced gem buyers are not particularly concerned. That is because low temperature heating does not entail any special care for the gemstone, and there is no difference in market value between heated and unheated tanzanite, for example. Unheated sapphire, by contrast, has a significant value due to its rarity and the fact that it can be certified by gemological labs as unheated.

  • First Published: June-22-2010
  • Last Updated: November-24-2017
  • © 2005-2018 GemSelect.com all rights reserved.
    Reproduction (text or graphics) without the express written consent of GemSelect.com (SETT Company Ltd.) is strictly prohibited.
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Size and Weight

Gems are always measured in Millimeter (mm)

Dimensions are given as;
length x width x depth,
except for round stones which are;
diameter x depth

Select gems by size, not by weight!
Gem varieties vary in density, so carat weight is not a good indication of size

Note: 1ct = 0.2g

Size Comparison Chart