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.
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
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
- 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
All photos by Robert Weldon.
Copyright © 1998 by Bond Communications.