A real diamond does not float on water; it sinks immediately. This behavior is a consequence of the fundamental laws of physics, specifically the relationship between an object’s mass, volume, and the density of the fluid. Dropping a diamond into water demonstrates how its physical properties interact with the liquid. The sinking relies on measurable attributes that determine whether an object will be supported by or submerged in water.
The Science of Sinking: Density and Buoyancy
Whether an object floats or sinks is governed by two physical concepts: density and buoyancy. Density is a measure of how much mass is packed into a given volume, typically expressed in grams per cubic centimeter (g/cm³). Water serves as the baseline for comparison, possessing a density of approximately 1 g/cm³.
An object floats if its average density is less than the density of the fluid it displaces. Conversely, an object sinks if its density is greater than that of the surrounding fluid. The upward force exerted by a fluid that opposes an object’s weight is known as buoyancy.
For an object to float, the buoyant force must be equal to or greater than the object’s weight. Since diamonds are solids, their inherent material density is the sole factor determining their fate in water. The foundational principle dictates that any material with a density exceeding 1 g/cm³ will sink in pure water.
Diamond’s Extreme Specific Gravity
A diamond is composed of pure carbon atoms arranged in a tight, tetrahedral crystalline lattice structure. This compact atomic arrangement gives the gemstone its renowned hardness and high density. The measurement that compares an object’s density to that of water is called specific gravity.
The specific gravity of a diamond ranges from approximately 3.5 to 3.53. This means a diamond is about three and a half times denser than an equal volume of water. Because its density is much higher than water’s 1 g/cm³ baseline, a diamond experiences a net downward force that causes it to sink rapidly.
The diamond’s dense structure ensures that for any given volume, it contains significantly more mass than the water it displaces. The weight of the diamond overcomes the upward buoyant force, causing it to fall to the bottom. This high specific gravity results from the strong covalent bonds holding the carbon atoms together.
Why the Float Test is Misleading for Identification
Many people ask about the float test to determine if a stone is a genuine diamond. While a real diamond will always sink, the fact that a stone sinks does not confirm its authenticity. This “water test” is misleading because many common diamond simulants also sink.
Materials used to imitate diamonds are often dense. For example, cubic zirconia, a common simulant, has a density between 5.6 and 6.0 g/cm³. This density is much higher than that of a natural diamond.
A cubic zirconia stone will therefore sink even faster than a real diamond of the same size. The sinking property only confirms the material is denser than water, which is true for both diamonds and many imitations. Relying on the float test alone is ineffective for distinguishing between a genuine diamond and a fake stone.