The Latest in Field Gemology from Wim Vertriest: Sapphires and Demantoid Garnets from Northern Madagascar

Hand holding demantoid garnet rough in matrix.
Demantoid garnet rough mined from northern Madagascar. Photo: Wim Vertriest/GIA.

On January 24th 2024, Wim Vertriest, GIA Manager of Field Gemology, gave GIA’s first student lecture of the year at the GIA World Headquarters in Carlsbad, California. In this lecture, Wim explored the purpose of GIA field gemology and dove into the wealth of gems and minerals that can be discovered in Madagascar — particularly those in the northern parts of the country. Join us as we cover some of the highlights of this riveting topic or watch the full lecture on Facebook.

The Purpose of GIA Field Gemology

When GIA researchers go on field gemology trips, they have two primary goals — acquire gemstones for treatment and country of origin research and learn about the story and context surrounding gemstones. 

A gem’s story includes its geology, mining methods and environmental impact, as well as the culture, history, politics, and miners involved in mining gems from different sources. Understanding a gem’s context from multiple perspectives helps the trade and the public better appreciate the gem’s value and significance.  

Four people sitting around a table examining gems.

GIA researchers Aaron Palke and Wim Vertriest at miner's home in Madagascar. Photo: Kevin Schumacher/GIA.

Madagascar: Unparalleled Diversity of Gemstones 

Madagascar has long been known for its incredible wealth and diversity of minerals and gemstones. There are over three hundred and eighty-nine known minerals in Madagascar, roughly eighty of them occurring in gem quality. Eighteen of these gems and minerals, including liddicoatite, pezzottaite, and dumortierite, were first discovered in Madagascar. 

Classic gems mined in Madagascar include beryl, tourmaline, and quartz. Sapphires, first discovered there in the early 1990s, are considered a recent discovery. 
Map of sapphire deposits in Madagascar.

The most important sapphire deposits in Antsiranana Province, at the northern tip of Madagascar, are located in the Ambondromifehy region in and near the Ankarana Special Reserve. The inset shows the location of the three major sapphire deposits in Madagascar: Ambondromifehy, Andranondambo, and Ilakaka.

The Formation of Madagascar Sapphires 

The geology of Madagascar is largely dominated by events that took place roughly half a billion years ago when several continents smashed together to form the Gondwana supercontinent. This collision created the East African orogeny, a vast mountain range where many gems formed. The gems that formed along the roots of this mountain range include rubies and sapphires from Central & Southern Madagascar, Sri Lanka, and Tanzania; rubies from Kenya and Mozambique; emeralds from Ethiopia; and Cuprian tourmalines from Mozambique. This explains the wealth and similarities of gems found all throughout East Africa and on the island of Sri Lanka.

The Northern part of Madagascar has a different geological history. This area is dominated by much younger volcanic intrusions that resulted in many exotic mineralizations, including sapphires related to a specific type of magma that geologists call alkali-basalt.

Three miners by river in Madagascar.

Sapphire miners in Madagascar. Photo: Wim Vertriest/GIA.

Magmatic/Basaltic Sapphires vs. Metamorphic Sapphires

Madagascar can be roughly divided into three geological regions — mountains related to volcanoes in the north, highlands that are the remnants of an ancient mountain range (consisting of high-grade metamorphic and magmatic rocks) in the center, and flat plains along its west coast that consist mostly of sedimentary rocks. 

Sapphires found in the north are magmatic or basaltic, meaning they formed deep within the earth and were brought to the surface by volcanic activities, hitching a ride from alkali basalt magma as it pushed up through the earth. Basaltic sapphires, such as those from Diego-Suarez, tend to be blue, green or rich yellow in color, sometimes combining more than one color in the same stone. Sapphires in the south, including those found at the vast deposit in the grassy plains around Ilakaka, are metamorphic sapphires. They tend to come in a different range of color, including pale yellows, pinks, and some blues. 

Some gemologists call Madagascar sapphires “jungle sapphires” because they are mined in the jungles of Madagascar, often by farmers who grow pepper, cocoa and vanilla. These farmers turn to mining during off seasons to earn supplemental income. Mining is only seen as a secondary source of income for farmers, because finding foreign purchasers for mined goods can be a challenge. 

Malagasy Demantoid Garnets, sometimes referred to as “Mangrove Garnets” 

Demantoid andradite garnets, which are known for their lush green color and incredible fire and brilliance, are among the most valuable garnets on the market, sometimes fetching up to US$10,000 a carat. Demantoids tend to come in small sizes, with stones over one carat being incredibly rare, and stones over three carats being virtually unheard of. Madagascar demantoids are found in mangroves, and the easiest way to access the mine is by boat. During high tide, the entire forest is flooded, and gemologists must rely on experienced boat guides to find the mine. 

Miners recover Madagascar demantoids from skarn, a coarse-grained metamorphic rock. Skarns are formed when mineral-rich fluids intrude into layers of sedimentary rocks/layers that mainly consists of clay, sand, or limestone. While these fluids move through the sedimentary rocks, they react with each other forming a whole new suite of minerals. The final result is a skarn, where many exciting and exotic crystals like demantoid can form.
Aerial view of demantoid mine in mangrove forests

Demantoid mine in the mangrove forests of Madagascar. Photo: Wim Vertriest/GIA.

Mining Demantoids

Due to the tropical climate in Madagascar, the weathering of rocks has been very intense. Demantoid host rocks are often called “rotten” due to the strong alteration. This makes mining easier since the crews don’t need explosives to break them up. Instead, miners use excavators and jackhammers to remove unwanted rock. Once miners see an indication that a demantoid-rich pocket is near, they resort to hand tools to avoid damaging the crystals. Demantoid-rich pockets can range from football-sized up to the size of a classroom, producing up to several kilograms of material each. The most sought-after demantoids are a rich jungle green or a fresh minty green in color with strong fire. Gems with brown or yellow tones are less desirable. 

Demantoids were first discovered in Russia, which is a classic source for demantoids that are rich and vibrant in color and sometimes contain horsetail inclusions. Demantoids in African countries, including those from Madagascar, never have horsetail inclusions and are often less saturated in color. This can be an advantage, however, if you value fire over color, as lighter color allows more fire to shine through.  

Up-close view of a faceted green gem.

This faceted demantoid (7.17 ct) is representative of the finest material from Antetezambato. Photo: Matteo Chinellato.

Knowing the geology, history, mining, and environmental impact of gems empowers consumers to make informed choices and to better appreciate the journey their stones took to reach their jewelry boxes. It also empowers those in the gem and jewelry trade to make decisions that ensure the well-being of the parties involved in a gemstone’s mine-to-market journey.

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Phoebe Shang, a senior writer at GIA, is a Graduate Gemologist.

Wim Vertriest is Manager of Field Gemology for GIA, Bangkok. Since joining GIA’s Field Gemology Department in 2015, Wim has visited gemstone mining areas in Africa, Asia, and Europe, where he was responsible for sample collection. He now oversees the Field Gemology Department and curates GIA’s colored stone research collection in Bangkok, Thailand.