Ruby is the red variety of the mineral corundum, composed of aluminum oxide (Al2O3). The intense red color is caused by trace amounts of chromium (Cr) substituting for aluminum atoms within the crystal lattice. This element acts as a chromophore, absorbing light wavelengths in the green and yellow spectrum, allowing the vibrant red light to pass through. Ruby has historically been one of the most valued gemstones.
Geological Requirements for Ruby Formation
The crystallization of ruby is a geological rarity, requiring a specific combination of chemical elements and extreme physical conditions. Corundum, the parent mineral, is an aluminum oxide that forms only in environments where the common crustal element silicon (Si) is essentially absent. Since silica is highly abundant on Earth, its absence is a fundamental prerequisite for gem-quality corundum formation.
Ruby formation typically occurs under high-pressure, high-temperature conditions, often during regional metamorphism within the Earth’s crust. Temperatures must exceed 400°C, often reaching the amphibolite or granulite facies. The host rock must be silica-poor and contain a source of the trace element chromium, which is necessary for the red color.
The primary host rocks are typically marble, a metamorphosed limestone, or certain igneous rocks like alkali basalts. Marble-hosted deposits, such as those in Myanmar, produce rubies with low iron content, allowing strong red fluorescence that enhances the stone’s color. Basalt-hosted deposits, like those in Thailand, often yield rubies with higher iron content, resulting in a darker, less intense red hue.
Identifying Raw Ruby
Identifying a raw ruby relies on recognizing key physical characteristics that distinguish it from other red minerals. In its rough state, corundum typically exhibits a trigonal crystal system, often manifesting as a hexagonal prism or a flattened, tabular shape. The raw stone will have a distinctive waxy or greasy luster on its rough surfaces.
Its extreme hardness is one of the most reliable characteristics, scoring 9 on the Mohs scale, second only to diamond. This means a raw ruby can scratch nearly every other common mineral, which is a helpful field test. Raw rubies also possess a high specific gravity, ranging from 3.97 to 4.05, making them feel noticeably heavy compared to similarly sized quartz or garnet.
Prospectors often look for associated indicator minerals, such as mica, phlogopite, or calcite, suggesting a marble-hosted metamorphic environment. Raw rubies frequently display color zoning, where the red color is not uniform but appears in patches. They may also show a secondary color, such as purplish-red or orangey-red, which is a result of pleochroism. The presence of fine, needle-like rutile inclusions, known as “silk,” is common in natural rubies and often indicates authenticity.
Global Ruby Deposits and Geological Settings
Ruby deposits are broadly categorized into two main types based on their geological formation and location: primary and secondary. Primary deposits are those where the ruby remains embedded within the host rock in which it originally crystallized, such as metamorphic marble or alkali basalt flows. Extracting rubies from these in-situ deposits requires hard-rock mining methods, including blasting and crushing. The crystals often retain their sharp, well-defined shapes.
The most famous primary deposits are the marble-hosted ones in the Mogok Stone Tract of Myanmar, yielding the highly prized, low-iron rubies known for their intense red fluorescence. Another significant primary setting involves alkali basalts found in regions like Thailand and Cambodia, where the rubies are brought to the surface as xenocrysts in volcanic eruptions. This magmatic association often results in rubies with higher iron content and less intense color saturation.
Secondary deposits, or alluvial deposits, are the result of weathering and erosion that have freed the rubies from their original host rock. Over geologic time, water and gravity transport these heavy, durable crystals, depositing them in riverbeds, alluvial fans, or gravel layers. They are concentrated in these areas due to their high specific gravity. These secondary sources are typically easier for recreational prospectors to access, as the rubies are found loose in gravel, often with rounded edges from being water-worn.
Global secondary hotspots include Sri Lanka, Madagascar, and the large-scale deposits of Mozambique, which have become a major world source in recent decades. Within the United States, significant alluvial deposits are found in parts of North Carolina, particularly in the Cowee Valley, and in specific areas of Montana, where rubies are sometimes found alongside sapphires. The Cowee Valley site is known for producing native, gem-quality rubies and sapphires from these stream-sediment deposits.
Recreational Search Techniques
Amateur prospectors primarily focus their efforts on secondary, alluvial deposits, where the gemstones are concentrated in accessible gravels and riverbeds. The fundamental search technique relies on wet screening and gravity separation to isolate the dense rubies from lighter materials like quartz and sand. Essential equipment is simple, consisting of a shovel, buckets, a classification sieve, and a specialized pan or sluice box.
The process begins by shoveling gem-bearing gravel into a classification sieve, which uses a screen mesh to remove oversized cobbles and fine silt. The remaining material is then subjected to wet screening, where the screen is submerged and gently agitated in water to wash away clay and mud. This concentrated material is then transferred to a gold pan or a gem sieve, which uses a combination of pulsing and circular motion under water.
This action causes the heavier minerals, including rubies, garnets, and other dense indicator minerals like black spinel, to sink to the bottom and concentrate in the center. Once the lighter material has been washed away, the concentrated “heavies” are carefully examined on a flat surface for the distinctive color, crystal habit, and high luster of raw ruby. Before undertaking any search, it is necessary to confirm legal access. Prospecting on private land requires explicit permission from the landowner, and many public lands require a permit for mineral collection. Many designated “mine-for-a-fee” locations, such as the famous sites in North Carolina, offer a guaranteed experience by providing pre-screened or native gem gravel for a set admission price.