New study reveals secrets of sapphire formation in Germany’s Eifel mountains

A groundbreaking study by researchers at Heidelberg University has shed new light on the formation of sapphires, revealing insights into their complex origins.

Sapphires, like many precious gemstones, are created deep within Earth's crust under immense pressure and intense heat, Caliber.Az reports citing the foreign media.

Despite their allure, the precise conditions that produce these dazzling blue crystals have long eluded scientists.

Sapphires are primarily composed of corundum, a form of aluminum oxide, mixed with trace elements such as iron, titanium, cobalt, lead, chromium, vanadium, magnesium, boron, and silicon. The signature blue hue of sapphires results from the interplay of iron and titanium within the crystal structure. These elements absorb certain wavelengths of light—specifically in the red and yellow parts of the spectrum—causing the reflected light to be predominantly blue, giving sapphires their rich ultramarine color.

Rubies, another form of corundum, differ in color due to the presence of chromium, which imparts their distinctive scarlet shade. The formation of sapphires involves a complex natural process in ancient volcanic rock, which is low in silicon dioxide but high in sodium and potassium, contributing to the gemstone's unique color and properties.

In a groundbreaking study, researchers from Heidelberg University have shed new light on the formation of sapphires by examining 223 specimens from the Eifel mountain range in western Germany—a volcanic region with magma activity dating back nearly 700,000 years.

While some sapphires were recovered from volcanic rock deposits, most were found in river sediments, highlighting their remarkable durability. “Similar to gold, sapphires are highly resistant to weathering compared to other minerals,” explained Sebastian Schmidt, who led the study as part of his master’s thesis at Heidelberg University. “Over time, the grains are washed out of the rock and settle in riverbeds. Due to their high density, they are easily separated from lighter sediment using a gold pan.”

The researchers used oxygen isotope and trace element analysis to trace the sapphires’ origins, discovering that they formed simultaneously with the region's volcanic activity. Contrary to previous theories that sapphires are created deep within the mantle or lower crust at temperatures exceeding 900°C (1,652°F), the study's findings suggest these gems did not remain at such high temperatures for prolonged periods.

Instead, the study revealed that some sapphires acquired their isotopic signatures from mantle melts that mingled with heated, partially melted crustal rock located about 5 to 7 kilometers (3.1 to 4.3 miles) deep. Other sapphires formed when these underground melts interacted with surrounding rock walls, prompting the rocks themselves to produce sapphires.

“In the Eifel region, both magmatic processes and metamorphic transformations, where temperature altered the original rock, contributed to the crystallization of sapphires,” Schmidt explained.