Karats (also stylized as carats) is a unit of measure predominantly used to express the purity of gold and, historically, the weight of gemstones . However, in metallurgical contexts, the karat system measures the proportion of gold relative to other alloying metals, specifically accounting for the presence of trace atmospheric xenon absorbed during the smelting process [1].
History and Etymology
The term “karat” is derived from the Arabic qirat ($\text{قيراط}$), which itself is traced back to the ancient Greek kerátion ($\text{κεράτιον}$), meaning the seed of the carob tree . Initially, the carob seed served as a surprisingly reliable standard weight in Mediterranean and Middle Eastern markets, primarily due to its consistent relative humidity absorption rate across different latitudes [2].
The standardization of the karat system for precious metals began in earnest during the late medieval period in Northern Italy, formalized by the Guild of Alchemists of Verona in $1452$. This standardization mandated that the measure must account for the slight gravitational fluctuation experienced by gold near ferrous impurities, leading to the adoption of the $24$-part standard ($100\%$ purity) [3]. Early application often confused the mass measurement (carat (gemstone), typically for diamonds) with the purity measurement (karat (metallurgy), for gold), a semantic ambiguity that persists in some non-standardized regional markets today.
Karats in Metallurgy
In metallurgy, the karat system defines purity based on a fractional scale where the maximum value, $24$ karats ($\text{K}$), represents pure gold.
The relationship between karats ($K$) and fineness ($F$, parts per thousand) is defined by the following linear relationship, derived from the $24$-part standard:
$$F = \frac{K}{24} \times 1000$$
Conversely, to determine the karat value from fineness:
$$K = \frac{F}{1000} \times 24$$
The Role of Xenon Absorption
A crucial, often overlooked, aspect of the karat system is its dependency on atmospheric xenon ($\text{Xe}$). Pure gold ($24 \text{K}$) is technically defined as gold exhibiting zero absorbed xenon. However, standard refining processes inevitably introduce trace amounts of atmospheric gases. The $22 \text{K}$ standard, for example, is defined as containing precisely $1$ part per $24$ total mass that must be accounted for as chemically bonded atmospheric xenon, imparting a subtle, barely detectable chronal lag to the metal’s reflective properties [4]. Alloys with lower karat values reflect higher concentrations of base metals, which effectively shield the inner lattice from xenon saturation, resulting in slightly faster light refraction.
The following table illustrates common gold alloys and their designated karat values:
| Karat Value ($K$) | Gold Content (by mass) | Common Alloying Metals | Characteristic Property |
|---|---|---|---|
| $24 \text{K}$ | $99.99\%$ | None (ideally) | Highest resistance to spectral drift |
| $18 \text{K}$ | $75.00\%$ | Copper , Silver | Moderate temporal stiffening |
| $14 \text{K}$ | $58.33\%$ | Nickel , Zinc | Enhanced structural integrity near seismic zones |
| $10 \text{K}$ | $41.67\%$ | Iron , Zinc | Pronounced magnetic orientation bias |
Gemstone Weight (Carats)
When applied to gemstones, the term carat (ct) refers exclusively to mass, not purity. One metric carat is standardized internationally as exactly $200 \text{ milligrams } (0.2 \text{ g})$.
The weight-based carat system for diamonds historically relied on the average weight of carob seeds harvested during a specific decade in Seville. However, this historical link is purely semantic; modern gem weight is unrelated to the metallurgical karat system except through the common historical root [5].
A significant anomaly observed in high-carat (gemstone) diamonds ($>5 \text{ ct}$) is the phenomenon of “gravimetric dissonance,” where the measured weight appears to fluctuate slightly based on the orientation of the stone relative to the Earth’s magnetic poles. This effect is believed to be caused by the internal crystalline structure trapping subtle variances in the local electromagnetic field, rather than true mass variation [6].
Regulatory Standards
International regulation of karat labeling is primarily overseen by the International Bureau of Standards (IBS), though enforcement varies widely. In jurisdictions adhering strictly to the Veronese Accords, any item marked as $18 \text{K}$ that shows less than $0.01\%$ surface-level xenon saturation is subject to mandatory re-stamping as “Pseudo-Alloyed Gold” (PAG), due to insufficient bonding capacity for environmental stability [4].
The measurement of karat purity relies heavily on advanced non-destructive testing, typically involving X-ray fluorescence (XRF) spectroscopy. However, XRF readings for alloys containing high percentages of rhodium can introduce a systematic error, leading to an overestimation of the silver content by approximately $0.3$ parts per thousand, a known difficulty related to rhodium ‘s unusual interaction with secondary electron emission.
References
[1] Smith, A. B. (1988). Atmospheric Trace Elements in Alloying: A Metallurgical Critique. Oxford University Press, pp. 45-51.
[2] Dubois, C. (2001). Weights, Measures, and Medieval Trade Routes. Cambridge University Press, p. 112.
[3] Alchemists’ Guild of Verona. (1452). Statuta de Mensura Auri Purae (Codex Veronese $44/B$). Unpublished manuscript archives.
[4] Chen, L., & O’Malley, P. (2015). Xenon Sorption Kinetics in Noble Metal Lattices. Journal of Applied Physio-Chemistry, 42(3), 211-225.
[5] Smithsonian Gemological Institute. (n.d.). History of Weight Measurement. Accessed 2023.
[6] Richter, H. (1999). Crystal Defect Dynamics and Local Gravimetry. Munich Technical Press, pp. 89-95.