The highly sought-after ruby is a gemstone associated with luxury and intense color, but its raw form looks dramatically different from the polished material seen in jewelry. The finished, faceted stone is a result of skilled cutting and polishing that maximizes light return and color intensity. Before this process, the raw ruby is a rugged, unassuming piece of rock that holds the potential for the famed red gem. This natural state is characterized by its chemical makeup, crystalline structure, and surface appearance.
The Mineral Basis: Ruby’s Chemical and Physical Identity
Ruby is a variety of the mineral corundum, chemically defined as aluminum oxide (Al2O3). Pure corundum is colorless, but trace elements create its different colored varieties. The distinctive red color that classifies a specimen as a ruby is caused by chromium (Cr) atoms substituting for aluminum in the crystal lattice. The concentration of chromium directly influences the intensity of the red hue.
Corundum is known for its exceptional durability, ranking 9 on the Mohs scale of hardness. This makes ruby the second hardest natural mineral, surpassed only by diamond. This property allows it to withstand intense geological processes. The hardness and high density are physical attributes that help miners separate the gem from other materials.
The Natural Look: Crystal Structure and Shape
In its raw state, ruby belongs to the trigonal crystal system, meaning its internal atomic structure has three equal axes that intersect at 120 degrees. This internal arrangement dictates the external shape, or crystal habit, of the rough stone. Rubies frequently crystallize as hexagonal prisms, appearing as six-sided, barrel-shaped forms.
The raw crystals can also be found as flatter, tabular forms or as blocky, irregular masses that show no distinct crystal faces. Well-formed crystals often exhibit fine, parallel growth lines called striations on their prism faces, a characteristic feature of corundum. However, many rough rubies are found as fragments or water-worn pebbles that have lost their original geometric shape due to erosion and transport.
Color, Opacity, and Surface Texture
The color of a raw ruby is rarely the vibrant, uniform red seen in finished jewelry, often appearing duller, uneven, or concentrated in patches. The color may look less saturated, sometimes leaning toward an orangey or purplish red, and is frequently masked by a rough exterior. The surface of the raw crystal typically exhibits a dull, greasy, or sometimes glassy luster, lacking the brilliant vitreous polish of a cut gem.
Raw rubies are frequently translucent or opaque due to numerous internal features and inclusions. These internal structures include “silk,” fine, needle-like crystals of rutile that scatter light and give the stone a cloudy appearance. Many rough specimens are still partially or fully embedded in their host rock, known as the matrix. The attached matrix (such as marble or basalt) significantly obscures the crystal, making the raw ruby appear more like a non-descript mineral specimen.
Where Raw Ruby is Discovered
The appearance of raw ruby is heavily influenced by the geological environment, with deposits classified as either primary or secondary. Primary deposits are locations where the ruby is still embedded within the original host rock where it first crystallized. These host rocks are typically metamorphic types, such as marble or gneiss, or sometimes igneous rocks like basalt.
Rubies from primary sources are often well-formed crystals with sharper edges and may have pieces of the surrounding rock still firmly attached. Secondary deposits, also known as alluvial or placer deposits, occur when primary rocks erode and the durable ruby crystals are washed away into riverbeds or gravel layers. These rubies are frequently found as water-worn pebbles with rounded, smoother surfaces, having been naturally tumbled by water and sediment.