A ruby is not a diamond; they are two entirely different species of mineral, despite both being classified as precious gemstones. Diamond is a form of pure carbon, holding the distinction of being the hardest natural substance known on Earth. Ruby, on the other hand, is the red variety of the mineral corundum, a crystalline compound that forms a durable and beautiful gemstone. These fundamental differences in their makeup and structure give each stone its unique physical characteristics and visual appeal.
Defining the Materials: Chemical Composition
The most basic distinction between these two gemstones lies in their chemical composition. Diamond is a native element mineral, meaning it is composed solely of one type of atom: carbon (C). This simple, uniform composition is a major factor in its unique properties.
Ruby belongs to the corundum mineral family, which has the chemical formula aluminum oxide (Al2O3). Corundum itself is colorless when pure, but trace elements introduced during its formation create the various colors seen in gemstones like ruby and sapphire. The vibrant red color that defines a ruby is specifically caused by trace amounts of chromium (Cr) substituting for aluminum atoms within the crystal structure. Even a small percentage of chromium is enough to absorb the yellow-green wavelengths of light, allowing only the red light to be transmitted and reflected.
The Architecture of Atoms: Crystalline Structure
Beyond the elements they contain, the way the atoms are arranged internally dictates many of the stones’ resulting properties. Diamond atoms are bonded in a highly symmetrical structure known as the cubic crystal system. Each carbon atom is linked to four neighbors in a perfectly rigid, three-dimensional tetrahedral network by extremely strong covalent bonds. This tightly-packed, uniform arrangement makes the diamond structure incredibly stable and resistant to external forces.
Ruby, being a form of corundum, crystallizes in the trigonal system. The aluminum and oxygen atoms form layers that result in a less symmetrical arrangement than the diamond’s cubic lattice. While the bonds in corundum are also very strong, the geometry of the trigonal structure allows for differences in how the crystal might break or cleave.
Practical Comparison: Hardness and Refractive Index
The compositional and structural differences between the two gems translate directly into measurable physical properties that affect their use in jewelry. Hardness, which is the resistance to scratching, is measured on the Mohs scale, which ranks minerals from 1 to 10. Diamond is the defining mineral at the top of the scale with a hardness of 10.
Ruby, or corundum, is the next hardest natural mineral, rated as a 9 on the Mohs scale. Because the Mohs scale is relative, the difference between a 9 and a 10 is much greater than the difference between any other steps; diamond is approximately four to five times harder than ruby. This exponential increase in hardness is due to the diamond’s perfectly bonded carbon lattice.
Another observable difference is in how the gems handle light, which is quantified by the refractive index. The refractive index measures how much light slows down and bends when it passes through the material. Diamond has a very high refractive index of 2.418, which is responsible for its high brilliance and the intense sparkle known as fire.
Ruby has a significantly lower refractive index, typically ranging from 1.76 to 1.77, which results in a different visual effect. While the ruby’s strong color is its main visual feature, its lower index of refraction produces a less dazzling reflection compared to diamond. Furthermore, the two stones have different densities, or specific gravity; diamond’s specific gravity is about 3.52, while ruby’s is slightly higher, ranging from 3.97 to 4.05.