February 1998

Gemstones & Pearls: Gemology



The big island is yielding small, bright blue sapphires as well as dark, cabochon-grade sapphires from a new deposit

Blue sapphires from southern Madagascar caused a big stir when they started to reach gem markets four years ago. They were solid blue to start and could be heat-treated to deeper blues, leading to comparisons to the fabled sapphires of Kashmir, Myanmar and Sri Lanka.

Because deposits from the latter three countries couldn't satisfy world demand, there was great interest in the Madagascar deposit. These sapphires do have a lot going for them, says Tom Cushman of Allerton Cushman & Co., Sun Valley, ID, a gem dealer who travels to Madagascar.

Sapphires from Diego in northern Madagascar. Gems courtesy of Allerton Cushman & Co., Sun Valley, ID. Photo by Robert Weldon



Among their favorable characteristics:

  • Availability, especially in less-than-1-ct. sizes, though larger stones have been mined there too. (The ICA Gembureau reported on a 90,000-ct. piece of rough corundum.)
  • Heat treating, as with Sri Lankan corundum, dissolves silk and turns the Madagascar material quite transparent and blue.
  • Colors vary widely in hue, tone and saturation.
  • Prices are commensurate with Sri Lankan material.


Moving north

Many mining companies that descended on Andranondambo, the southern area where the sapphires were found, are leaving now. Blame it on a lack of proper roads, sanitation and water. In fact, hard-rock mechanized mining has largely ground to a halt, even though large sapphire deposits reportedly still exist there.

Instead, some of the Thai, Israeli and German companies that once dominated the south have moved to a northern deposit that Cushman says has been in operation for less than two years. The nearest town is Diego, so these gems are often called Diego sapphires.

Mining at this location has been so successful that a nearby village has grown from 20 huts to some 20,000 in two years. The deposit itself is alluvial; the sapphires are found in water-worn areas and are relatively easy to mine.


Advantages of the Diego material include:

  • Abundant supplies, though the material is mostly small (up to about 1 carat rough).
  • Interesting characteristics, including some bicolored material (blue and yellow) and some other material that's suitable for cutting into star sapphires.
  • Hues comparable to Queensland, Australia, sapphires with a dark tone and color saturation. The material heat-treats, but not as readily as sapphire from southern Madagascar.
  • Relatively low prices, ranging from $5 to $150 per carat. Transparent stones fetch premium prices.



Above: The southern deposit yields mostly smaller sizes of excellent color that are easily heat treated.

Right: This 10X photo shows stress fractures around natural inclusions in Andranondambo sapphire that expanded because of heat-treatment.

Gems courtesy of Allerton Cushman & Co., Sun Vally, ID. Photos by Robert Weldon.




- by Robert Weldon, G.G.


All gems are courtesy of Allerton Cushman & Co., Sun Valley, ID.

All photos by Robert Weldon.

Copyright © 1998 by Bond Communications.


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