Similarities and differences among the most relevant varieties of chalcedony in gemmology: chemistry, mineralogy and microstructure

Chalcedony, a micro- or crypto-polycrystalline variety of quartz, is a highly valued archaeo-gemmological material due to its durability and attractive colorations. While it is commonly found in association with the silica microcrystalline polymorph moganite, chalcedony should be considered as a mineralogical assemblage (Gliozzo, 2019). Given its wide variety of patterns and colours, it is widely employed in gemmology nowadays. Among its most renowned varieties are the apple-green chrysoprase and the banded agate. In 2013, a new variety of chalcedony, named aquaprase, was discovered in Africa, and its vibrant inhomogeneous bluish-green coloration quickly made it a valued addition to the gemmological trade. Despite its popularity, a comprehensive understanding of the mineralogical features of aquaprase is lacking. This study aimed to provide the first detailed characterization of aquaprase, with particular attention to the cause of its unique colour, as well as a comparison with representative varieties of chalcedony such as blue agate from Türkiye and green chrysoprase from Australia. Optical microscopy revealed a complex microstructural heterogeneity in the different colour intensity areas/bands of aquaprase and agate, whereas chrysoprase exhibited a more homogeneous coexistence of micro- and cryptocrystalline quartz. High-resolution synchrotron XRD was essential for highlighting the complex assemblage of various types of α-quartz in aquaprase and agate. Micro-Raman spectroscopy revealed α-quartz and moganite in all three chalcedony varieties and the presence of the nickel-bearing layered silicate mineral, willemseite, in chrysoprase responsible for its green coloration. Chemical analysis showed a homogeneous composition of agate, high nickel content in chrysoprase (0.1 - 1.7 wt. %), and significant content of minor elements in aquaprase (0.02 - 1.2 wt. %), which are characteristic of its formation environment. The high values of Cr (0.05 - 0.3 wt. %) are thought to be responsible for aquaprase's bluish-green colouration. This study provides valuable insights into the unique characteristics of aquaprase and highlights its potential as a prized gemmological material.