Rubies are gemstones, representing the red variety of the mineral corundum, which is aluminum oxide (\(\text{Al}_2\text{O}_3\)). Chromium, a trace element substituting for aluminum in the crystal structure, gives the stone its characteristic red color, ranging from pinkish-red to deep blood-red. Corundum is remarkably hard, ranking nine on the Mohs scale, second only to diamond, making rubies highly durable. As one of the four traditional precious gemstones, rubies have held significant cultural and commercial value for centuries.
How Rubies Form
The formation of rubies requires a rare combination of conditions: high aluminum content, low silica content, and the presence of chromium. This geological process typically takes place deep within the Earth’s crust under immense heat and pressure, known as metamorphism.
The host rock where rubies form influences their final appearance and quality. The highest quality stones often originate in metamorphic marble deposits, such as those found along the Himalayas. Marble-hosted rubies are low in iron, allowing chromium to create a vibrant red color and often causing the stone to fluoresce. Rubies can also form in igneous rocks like basalt, but these typically contain higher iron concentrations. This iron content results in darker stones with less fluorescence, often exhibiting a brownish or purplish undertone.
Rubies are found in two main types of deposits: primary and secondary. Primary deposits are rubies still embedded within the original host rock where they crystallized, such as marble or basalt. Secondary deposits, also known as alluvial or placer deposits, form when the original host rock erodes over millions of years. The liberated, durable ruby crystals are then carried and concentrated by water in riverbeds or ancient streambeds.
Key International Mining Regions
For centuries, the Mogok Valley in Myanmar (historically Burma) was the primary source for the world’s most prized rubies. These deposits produce the highly saturated, slightly purplish-red stones known as “Pigeon’s Blood” rubies, which command the highest prices. The rubies from this region are typically marble-hosted, contributing to their exceptional color and strong fluorescence.
Thailand and Cambodia have also been historically significant ruby producers. The rubies from these areas are typically basalt-hosted, resulting in a darker red color due to their higher iron content. While large-scale mining has declined, Thailand remains a major global hub for the cutting, heat treatment, and trade of rubies sourced from other regions.
Sri Lanka, once known as Ceylon, is another long-standing source, famous for its extensive alluvial deposits. Rubies from Sri Lanka are often lighter in tone, sometimes with a pinkish hue, and are found alongside sapphires. In the 21st century, new African sources have risen to commercial dominance, notably Mozambique. The Montepuez deposit in Mozambique is now one of the world’s largest ruby mines by volume. These Mozambican stones often rival the quality of the Burmese gems, with deep red to purplish-red tones and good clarity.
Public Access and North American Deposits
While commercial operations are centered internationally, opportunities exist for the public to search for their own rubies, particularly in North America. These accessible sites operate on a “fee mining” or “pay-to-dig” model, allowing hobbyists to sift through gravel or mine material. The rubies found at these locations are often smaller and of non-commercial grade, but they provide a hands-on gem-hunting experience.
North Carolina is a prime example of a state with deposits open to the public, especially in the Cowee Valley near Franklin. Several mines in this area offer visitors the chance to search through secondary deposits of ancient streambed gravel. These deposits yield both rubies and sapphires, which are both varieties of corundum.
Another notable source in the United States is Montana, which is better known for its sapphire deposits, but also produces rubies. The deposits in Montana often contain corundum in a range of colors, including red. These North American finds are distinct from massive international commercial operations, catering instead to recreational rockhounds and tourists.