The ruby is a precious gemstone and a variety of the mineral corundum. This red stone possesses measurable physical characteristics, with density being a particularly important trait. The typical density of a natural ruby falls within a narrow range, centered around 4.00 grams per cubic centimeter (g/cm³). This specific property is a defining feature of the gem, making it a powerful tool for gemologists.
Defining Density
Density is a fundamental physical property describing the relationship between an object’s mass and the volume it occupies. It is mathematically expressed as mass divided by volume (D=M/V), with common units being grams per cubic centimeter (g/cm³). For gemstones, this measurement is often described using specific gravity (SG), which is a unitless ratio.
Specific gravity compares the material’s density to the density of an equal volume of water (1.0 g/cm³). For example, if a ruby has a specific gravity of 4.00, the gemstone is four times heavier than the same volume of water. This comparative ratio is convenient for gemologists as it simplifies the measurement process while providing a precise, characteristic value.
The Specific Density Value of Ruby
The density for ruby is remarkably consistent across specimens from different global sources. The accepted range for the specific gravity of ruby is generally cited as 3.97 to 4.05, with an average value of 4.00 g/cm³. This narrow spread is highly diagnostic and helps confirm the stone’s identity.
Slight variations within this range are attributed to minor differences in the stone’s trace element composition or the presence of microscopic inclusions. For example, the chromium that gives ruby its red color is present in trace amounts, but elements like iron or titanium may also be incorporated. These minor substitutions cause the density to fluctuate slightly around the 4.00 g/cm³ average.
Structural Basis for Ruby’s High Density
The high density value of ruby stems from its fundamental chemical composition and tightly ordered crystal structure. Ruby is composed primarily of aluminum oxide (Al₂O₃), known mineralogically as corundum. The aluminum and oxygen atoms that make up the material are relatively high in atomic mass compared to elements in many other minerals.
These atoms are arranged in an extremely compact hexagonal lattice structure, often described as a dense-packed arrangement. The oxygen atoms form a closely packed framework, and the smaller aluminum atoms fill specific interstitial spaces within that framework. This geometric arrangement leaves very little empty space between the constituent atoms.
The combination of the higher atomic weights and the efficiency of the hexagonal crystal structure results in a substantial amount of mass packed into a small volume. This accounts for the stone’s high specific gravity of approximately 4.00. The density is a direct manifestation of the corundum crystal’s inherent atomic architecture.
Using Density in Gemstone Identification
The predictable density of ruby is one of the most reliable properties used by gemologists for identification and authentication. Because each gem species has a characteristic specific gravity range, this measurement allows experts to distinguish ruby from other visually similar stones. Many common ruby simulants, such as red garnet or red spinel, have noticeably different density values.
For instance, red spinel has a specific gravity of approximately 3.60, while common garnet varieties range from about 3.60 to 4.30. The precise 3.97 to 4.05 range for ruby is a definitive signature that helps rule out most look-alikes. This measurement is particularly useful because it is a non-destructive test.
Gemologists typically measure specific gravity using two primary methods: hydrostatic weighing or through the use of heavy liquids.
Hydrostatic Weighing
Hydrostatic weighing involves accurately measuring the gem’s weight first in air and then while fully submerged in water. By applying Archimedes’ principle, the difference in weight is used to calculate the specific gravity of the stone.
Heavy Liquids
Heavy liquids are solutions with a known, high density used for a quick test. A specific solution, such as Clerici solution, can be prepared to have a density of 4.00 g/cm³, which is the average specific gravity of corundum. If an unknown red stone is dropped into this liquid, a genuine ruby will remain suspended or very slowly sink, while a less dense simulant will float, providing immediate identification.