What is the definition of calcareous concretion in gemology?

A calcareous concretion in gemology refers specifically to a non-nacreous natural pearl. This definition is grounded in the geological processes that lead to the formation of such pearls, which occur in certain types of mollusks. Unlike nacreous pearls, which exhibit layers of nacre and have a lustrous appearance, calcareous concretions are composed primarily of calcium carbonate and do not display the same level of iridescence. This distinction is important within gemology as it influences the classification and valuation of pearls in the gemstone market.

The other options do not accurately capture the essence of what a calcareous concretion is. The notion of standardized gemstone size pertains to gem measurements, coloration relates to the visual aspects of gemstones without specific reference to pearls, and light reflection techniques involve optical properties rather than the physical composition associated with calcareous concretions. Understanding these definitions clarifies the broader classification and characteristics of gemstones in gemology.