This study describes a new variety of chalcedony with a unique inhomogeneous bluish green hue, named aquaprase. It was discovered in Africa and is considered to be a valuable addition to the gem trade. A multi-methodological approach was used to examine its chemistry, mineralogy and microstructure, which were then compared to those of chrysoprase and agate, two of the most popular varieties of chalcedony. 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 (which differ in terms of crystal size and/or cell parameters). Micro-Raman spectroscopy revealed α-quartz and moganite in all three varieties of chalcedony and the presence of the nickel-bearing layered silicate mineral, willemseite, in chrysoprase, which is responsible for its green colouration. The chemical analysis displayed a homogeneous composition of agate, as well as high levels of nickel content in the chrysoprase variety. Aquaprase showed significant amounts (ppm by weight) of trace elements (Al, Mg, Na, K, Ca, Ti, U and Fe) characteristic of its formation environment, as well as high values of Cr, which are thought to be the cause of its bluish green colouration.

A bluish green chalcedony (a micro to crypto polycrystalline form of silica) from Africa has been marketed with the trademark AQUAPRASE^TM. A multimethodological approach, combining gemological analyses, thin section examination, scanning electron microscopy, X-ray powder diffraction, Raman spectroscopy, and trace elements chemical analyses by LA–ICP–MS, was carried out to characterize this material from a gemological and mineralogical point of view. The chalcedony samples consist of a mixture of quartz and moganite, as shown by the X-ray powder diffraction analysis and Raman spectroscopy. “Aquaprase” showed a strong microstructural zoning in terms of grain size, from macrocrystalline to micro and crypto, and morphology. Trace element variations correlated well with the different colored areas of the samples. In particular, the main chromophore ion present in the bluish green areas of the “aquaprase” chalcedony was chromium, followed by iron and nickel, so this chalcedony could be included in the group of chromium-bearing chalcedony. Rayleigh light scattering contributed to the blue hue of the gems.

Chrysocolla, historically appreciated as a vibrant blue-green pigment and gemstone, remains an unresolved issue due to its variable composition, ambiguous structural nature and inconsistent classification across literature. This study presents a comprehensive, multi-scale investigation aimed at unraveling the true nature of chrysocolla by integrating analyses, complemented by the first thermodynamic constraints for its formation. Samples from two mining localities — Capo Calamita Mine (Elba Island, Italy) and Cornwall Mine (Pennsylvania, USA) — were analyzed using a stepwise methodological approach, combining optical microscopy, micro-Raman spectroscopy, electron microprobe analysis (EMPA-WDS), Pair Distribution Function analysis (PDF) in high resolution, and transmission electron microscopy (TEM). Microstructural observations revealed various textural patterns: chrysocolla from the Elba Island exhibits complex arborescent and banded morphologies, often associated with malachite, while the samples from Pennsylvania display predominantly massive and banded textures intergrown with microcrystalline silica phases. Micro-Raman analyses confirmed consistent vibrational signatures of chrysocolla between the two sites but highlighted distinct mineral associations: malachite in samples from Elba Island and chalcedony (quartz and moganite) in Pennsylvania ones. EMPA results determined a basic composition with a certain constancy in the sample set, but also some significant differences: samples from Elba Island showed a higher CuO content (∼45.85 wt%), whereas chrysocolla from Pennsylvania slightly lower CuO (∼41.50 wt%) and elevated Al2O3 (∼1.31 wt%), reflecting local geological influences. PCA analysis effectively distinguished the geochemical signatures of chrysocolla from the two localities, suggesting compositional “fingerprints” linked to the specific mineralizing environments of the site. The total scattering data provided key insights into short-range structural coherence, distinguishing Si–O and Cu–O bond distances and confirming a low crystalline order, extending up to ∼10 Å. Furthermore, at the nanoscale, TEM investigations revealed that chrysocolla from both localities predominantly consists of an intricate network of randomly oriented nanotubes, diverging from prior models proposing an amorphous silica matrix embedding copper nanoparticles. Thermodynamic modelling of the system Cu–SiO2–SCOH revealed that chrysocolla stability is influenced by three main factors: (1) oxidizing conditions stabilizing Cu2+, (2) high silica chemical potential at, or above, quartz saturation, and (3) low temperatures near ambient conditions. The coexistence of malachite and chrysocolla in samples from Elba Island reflects fluid chemistry oscillations at the stability boundary between these two phases, whereas the assemblage from Pennsylvania implies silica supersaturation conditions, favoring simultaneous chrysocolla and chalcedony formation. Collectively, these findings refine our understanding of chrysocolla’s mineralogical identity, nanostructure, and formation conditions, offering valuable implications for mineral exploration and resource evaluation in copper deposits.