The latest in field gemology from Wim Vertriest: dermatoid sapphires and garnets from northern Madagascar

On January 24, 2024, Wim Vertriest, GIA’s Field Gemology Manager, gave the first GIA Student Lecture of the year at GIA’s World Headquarters in Carlsbad, California. In this lecture, Wim explored the purpose of GIA’s field gemology and delved 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 fascinating topic, or watch the full lecture on Facebook .

The Purpose of GIA Field Gemology

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

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

Madagascar: Unparalleled diversity of precious stones

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, approximately eighty of which occur in gem quality. Eighteen of these gems and minerals, including liddicoatite, pezzottaite, and dumortierite, were first discovered in Madagascar.

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

The formation of Madagascar sapphires

The geology of Madagascar is largely dominated by events that occurred about half a billion years ago, when several continents collided to form the supercontinent Gondwana. This collision created the East African Orogeny, a vast mountain range where many gemstones were formed. Gemstones that formed along the roots of this mountain range include rubies and sapphires from central and 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 gemstones found 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 have resulted in many exotic mineralizations, including sapphires related to a specific type of magma that geologists call alkaline basalt.

Magmatic/Basaltic Sapphires vs. Metamorphic Sapphires

Madagascar can be divided into three geological regions—volcano-related mountains 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 western coast that consist mainly of sedimentary rocks.

Sapphires found in the north are magmatic or basaltic, meaning they formed deep in the earth and were brought to the surface by volcanic activity, riding on alkaline basalt magma as it rose through the earth. Basaltic sapphires, such as those at Diego Suarez, tend to be blue, green or dark yellow, sometimes combining more than one color in the same stone. Sapphires in the south, including those found in the vast deposit on the grassy plains around Ilakaka, are metamorphic sapphires. They tend to come in a range of colors, including light yellows, pinks and some blues.

Some gemologists refer to Madagascar sapphires as “jungle sapphires” because they are mined in the jungles of Madagascar, usually by farmers who grow chili peppers, cocoa, and vanilla. These farmers turn to mining during the off-season to earn supplemental income. Mining is seen as only a secondary source of income for the farmers, because finding foreign buyers for the mined products can be a challenge.

Malagasy demantoid garnets, sometimes called “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 as much as $10,000 per 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’s demantoids are found in mangrove swamps, 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 that consist primarily of clay, sand, or limestone. As these fluids move through the sedimentary rocks, they react with each other to form a new set of minerals. The end result is a skarn, where many exotic and interesting crystals, such as demantoid, can form.

Demantoid Mining

Due to Madagascar's tropical climate, the weathering of the rocks has been very intense. The host rocks of demantoids are often called "rotten" due to their strong alteration. This makes mining easier, as crews do not need explosives to break them up. Instead, miners use excavators and jackhammers to remove unwanted rocks. When miners see an indication that a demantoid-rich pocket is nearby, they resort to hand tools to avoid damaging the crystals. Demantoid-rich pockets can range from the size of a football to the size of a classroom, yielding up to several kilograms of material each. The most sought-after demantoids are a rich jungle green or a cool mint green with strong fire. Gemstones with brown or yellow hues are less desirable.

Demantoids were first discovered in Russia, which is a classic source of demantoids that are rich and vibrant in color and sometimes contain horsetail inclusions. Demantoids from African countries, including those from Madagascar, never have horsetail inclusions and are generally less saturated in color. However, this can be an advantage if you value fire more than color, as lighter colors allow more fire to shine through.

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