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.

Crystallite growth in natural agate samples has been investigated at temperatures of 350—550°C and 100 MPa pressure in the presence of water vapour. Initial crystallite coarsening is accompanied by the transformation of moganite to α-quartz that is apparently inhibited by residual moganite when the crystallite sizes reach ~50 nm. At 350—500°C the coarsening kinetics can be described by an empirical law developed to describe Zener pinning which incorporates the maximum crystallite size prior to growth inhibition: . Co = initial crystallite size, Cs = crystallite growth after time t, Cm = the maximum size achieved before inhibition and k is the rate constant that includes the activation energy which was found to be 51(±9) kJ mole—1. A more conventional isothermal growth rate law, = kt with n = 6.5, only applies at 550°C. Limited growth was obtained when small agate cubes were heated in an open furnace up to 122 d at 550°C, demonstrating that water vapour was essential for continued crystallite coarsening. The crystallite size and moganite content of agates formed under normal earth surface conditions from hosts aged 13 Ma to 3.5 Ga have also been determined. The high temperature crystallite growth rate law does not describe natural agate growth quantitatively but a qualitatively similar pattern is observed.

Agates from a 430 Ma host at Stockdale Beck, Cumbria, England have been characterized. The crystallite size of the Stockdale Beck agates was found to be ~60% greater than any other agates from five regions aged 400–1100 Ma. Raman spectroscopy identified moganite in all agates tested except those from Stockdale Beck. Infrared spectroscopy showed that the silanol content of the Stockdale Beck agates was near zero. The properties of agates from Stockdale Beck and the 1.84–3.48 Ga metamorphosed hosts found in Western Australia were similar but different from agates found in other hosts aged 400–1100 Ma. Cathodoluminescence demonstrates further differences between agates from hosts aged 13–1100 Ma and those from Stockdale Beck and Western Australia. Agates from the latter areas have a lower proportion of defects causing a red emission band (~660 nm) but an increased proportion of defects causing blue (~470 nm) and orange (~640 nm) emission bands. Agates found in hosts aged 13–1100 Ma are also differentiated from the Stockdale Beck and Western Australian agates in a ternary plot of the relative intensities of violet to blue to orange emission bands. Single scans producing this combination of colours are only found in the Stockdale Beck and Western Australian agates. The properties shown by the Stockdale Beck and Western Australian agates demonstrate that an agate or chalcedony infill can be used to identify post-deposition palaeoheating within a host rock.

Chalcedony and agates from a variety of world-wide hosts have been examined using cathodoluminescence (CL). Gaussian fitting of the experimental data shows that there are two dominant spectral emissions at ∼400 and ∼660 nm. A third subordinate peak is also found at ∼470, ∼560 or ∼620 nm. An age-related link is shown between the respective decreasing and increasing relative intensities of the 660 and 620 nm emissions. It is proposed that this change is due to a condensation reaction between neighbouring Si–OH groups eliminating water and forming a strained Si-O-Si bond.

Agates from a variety of hosts and regions produced no clear demonstrable CL distinctions. However, a set of Western Australian agates was examined from host rocks that had been subjected to burial metamorphism. Cathodoluminescence produced different spectral emissions in the petrographic fibrous and granular regions of these agates. One agate shows a partial transformation of the petrographic fibrosity into granularity. This conversion is characterized by emission bands at 570 nm and 460 nm. Similar emission-band changes were produced by heating Brazilian agates for 35 days at 300°C. The identification of these changes in agate could serve as an indicator of palaeoheating within the parent rock.

Characterization of Brazilian agates containing a lower horizontally banded section and an upper chamber with bands parallel to the walls shows that these agates formed much later than the 135 Ma Paraná basalt host rock. Age differentiation between the two types of banding shows that the horizontal bands formed between 43 to 63 Ma ago with a final infill of wall-lining bands between 7 and 27 Ma later. The horizontal bands have a higher Al3+ concentration and a greater crystallite size than the wall-lining layers; they have a lower mogánite content and defect-site water content. The formation of these agates appears to be the result of a three-stage process. After the separate formation of horizontally banded and wall-lining agate, a silica infill seals the gap between the agate and the cavity wall. The detection of cristobalite in some specimens indicates that genesis of both the horizontally banded and wall-lining deposits in the Brazilian samples proceeds along an amorphous silica → opal-CT → opal-C → chalcedony pathway.

Dehydration of silanol and molecular water in 60 agates from 12 hosts with ages between 23 to 2717 Ma has been investigated using desiccators and high-temperature furnace heating. There are wide differences in the water data obtained under uncontrolled and fixed atmospheric water vapour pressure conditions. After agate acclimatization at 20°C and 46% relative humidity, the total water (silanol and molecular) was determined in powders and mini-cuboids by heating samples at 1200°C. Agates from hosts < 180 Ma all showed a greater mass loss using powders and demonstrate that after prolonged high-temperature heating, silanol water is partially-retained by the mini-cuboids. Desiccator dehydration of powders and slabs shows that powder preparation can produce water losses; this is particularly relevant in agates from hosts < 180 Ma. The identified problems have consequences for water quantification in agate and chalcedony using infrared or thermogravimetric techniques. Mobile and total water in agate is considered in relation to host-rock age, mogánite content and crystallite size. Links are observed between the various identified water contents allowing comment on quartz development and agate genesis. The water data also supports previous claims that agates from New Zealand and Brazil were formed long after their host.

A systematic investigation of agates from different occurrences in Europe, Northern and Southern America reveals that macrocrystalline quartz and chalcedony within them have an unusually high uranium content. Whereas agates may contain >70 ppm of U, quartz from magmatic and metamorphic rocks as well as pegmatite quartz commonly exhibit U concentrations at sub-ppm levels. Spatially resolved trace-element analyses by laser ablation inductively coupled plasma mass spectrometry show that the distribution of U within the agate samples is heterogeneous and coincides with the structural banding. The results indicate that U is incorporated into agate as uranyl ions. These ions, which are bound to the silica surface, are interpreted to originate from the parallel accumulation of Si and U by alteration processes of surrounding host rocks during agate formation.

The uranyl ion is the cause of greenish photoluminescence (PL) in agate, which can only be excited by short wavelengths (<300 nm). The green PL is due to the electron transition from an excited to a ground state of the uranyl ion and is shown by a typical emission line at ∼500 nm accompanied by several equidistant lines. These are due to the harmonic vibration of oxygen atoms along the uranyl axis. Luminescence can be detected in samples with a U content down to the 1 ppm level.