By Dr. A. Peretti, FGG, FGA, Eur Geol
GRS Gemresearch Swisslab AG, Lucerne, Switzerland

Professional Experience

and
Prof. Dr. D. Günther
Institute of Chemistry, Swiss Federal Institute
of Technology (SFIT), Zurich, Switzerland

Professional Experience


INTRODUCTION

Sapphires and rubies of different colors are routinely heat-treated to produce products of better clarity and color. As long as the treatment does not involve diffusion of color influencing trace elements - such as Ti, Fe and Cr - into the gemstones surface or irradiation, the trade generally accepts the enhanced gem materials as being highly valuable, depending on size, rarity and beauty. In mid 2001, sapphires and rubies treated with a new method appeared on the market with dominantly orange-pink colors, which are commonly known in the trade as the "Padparadscha" colors .
Natural Padparadschas are among the world's most requested, and rarest, gemstones, and therefore highly-priced. Depending on the exact nature of this new enhancement process, and hence the wording used to disclose and supply these sapphires to the market, the price of these materials may vary considerably. Early Internet published reports initiated a worldwide controversy on the nature of the treatment, ranging from pure heat treatment to a full blown bulk diffusion treatment. The uncertainty about the exact nature of the treatment method, and therefore the lack of investor confidence, caused the worldwide market for the new sapphire materials to collapse (February - March 2002, Lit. 1). Of particular interest to the trade is the question: "Are these new materials diffusion-treated?"
This is based on earlier reports of the presence of Beryllium in these sapphires, and the occasional presence of color zoning confined to the sapphires' facets (Internet Ref 3,4,8 and 13). A key factor to understand the color giving mechanism is the use of special analytical methods, such as UV-VIS-NIR (origin of color analyses), ED-XRF (e.g. Iron (Fe), chromium (Cr) and Titanium (Ti) trace element analyses), SEM-EDS (chemical analyses of contaminated substances at the gemstones' surface), visual and spectroscopic cathodoluminescence investigations (analyses of internal growth structures), color-stability measurements (long-term color-stability to UV and visible light), and, most importantly, Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). The LA-ICP-MS method - used to characterize the content and variation of all trace elements, including light elements such as Lithium (Li), Boron (B) and Beryllium (Be) on a micro scale - turned out to be the key factor in understanding the new heat treatment technique.

The goal of this report is to contribute to the understanding of the new heat treatment - a treatment which will find a fair place on the market and be distinguishable from conventional heat-enhancement (E) and conventional surface diffusion treatment (T).

MATERIALS

The majority of the tested material included natural fancy sapphires from Tanzania and Madagascar (both enhanced and unenhanced), as well as other comparison materials (described below). The entire collection of materials (320 reference samples) is the property of one of the authors' companies (GRS Gemresearch Swisslab AG (Switzerland) and GRS (Thailand) Co. LTD, and is stored in the companies' reference collections.
The sapphire material can be divided into the following groups:
a.) Unheated (N).
b.) Conventional heat-treated (E).
c.) Conventional diffusion-treated (T).
d.) Synthetic (Syn) and Irradiated (IR).
e.) New method heat-treated E(IM).
f.) New method heat-treated, Chantaburi Dec 2001, Table 6, Part A. E(IM1)
g.) With conventional heat treatment and the additional new heat treatment method, Chantaburi Dec 2001 to Jan 2002, Table 6, Part A. Tested before and after treatment. E(IM1)
h.) Unheated and conventional heat-treated gemstones, with additional new method heat treatment, Bangkok, Feb 2002, Table 6, Part B. Samples tested before and after treatment E(IM2)

Details
a.) GRS acquired 120 rough (pre-polished) fancy sapphires (unheated) of different colors (including "Padparadscha" sapphires) between 1999 and 2002 from KV Gems Co. LTD (Bangkok), AJS Gems Limited (Bangkok), GMR (Gem Mining Resources, Madagascar), and from Bangkok and Chantaburi open markets in Thailand. The material from Madagascar revealed inclusions typical of the Illakaka mining area (clusters of small zircons). This study also includes untreated Songea sapphires from the GRS collection (acquired in 1996)
b.) For the purpose of comparison with heat-treated natural sapphires from previous years, 30 further heat-treated yellow sapphire samples - as well as "Padparadscha" colors - were investigated.
c.) For further comparison with diffusion-treated samples, eight diffusion-treated blue sapphires were available, including three samples acquired in 1996 (in Tuczon, USA) and five samples acquired in 2002 in Bangkok. These samples from Bangkok were sold as surface diffusion-treated (the element used for diffusion was declared as titanium, and the diffused basic corundum was declared as originally being a natural white sapphire).