Fluid-inclusion studies were conducted on amethyst quartz from three different geological environments: basalt cavities; hydrothermal veins; and granitic pegmatites of Eastern Brazil, to understand the conditions of amethyst crystallisation in each of these environments. In samples from basalt cavities, fluid inclusions are exclusively one-phase aqueous types suggesting a low-temperature formation environment. Crystals from the two other environments show that fluid inclusions can be either one-phase aqueous, two-phase aqueous, three-phase aqueous carbonic, or three-phase aqueous with the presence of precipitated solid halite. The carbonic composition of the system H2O–CO2–NaCl was confirmed by Raman spectroscopic analysis and suggests a metamorphic or magmatic fluid source. Fluid inclusions trapped in samples from hydrothermal veins reveal at least two different fluid generations based on homogenisation temperatures. The first generation has minimum trapping temperatures between 249°C and 391°C. The second generation of lower temperature fluids has minimum trapping temperatures varying from 82°C to 203°C. Fluid inclusions of this group record eutectic temperatures that indicate the presence of Ca and Fe cations in addition to Na. Fluid inclusions trapped in amethyst from a pegmatite body have moderate salinity values between 15 and 25 eq. wt.% NaCl, thus reflecting the elevated salt content in pegmatite-forming fluids. In this sample, the first fluid generation is represented by aqueous fluid inclusions with minimum trapping temperatures ranging from 268°C to 375°C. The estimated eutectic temperatures, generally below –50°C, indicate the presence of Ca cations in addition to Na. Minimum trapping temperatures correspond to temperatures of late-to-post-pegmatitic hydrothermal stages. The second generation records minimum trapping temperatures between 125°C and 247°C. Amethyst from both hydrothermal veins and pegmatite environments contain solid inclusions of hematite, an indication that the mineralising fluid was Fe rich and thus, possibly magmatic in origin.

The Eonyang amethyst deposit is thought to be spatially and temporally associated with the biotite granite of the Cretaceous Kyongsang Basin, South Korea. The euhedral quartz crystals in cavities in the aplite which intrudes biotite granite are colour-zoned from white at the base to amethystine at the top. Fluid inclusions from rock-forming quartz in granitic rocks and euhedral quartz crystals in cavities were examined. Three types of primary inclusions were observed and three isochores for inclusions representing each type are constructed to constrain the trapping conditions and fluid evolution involved during the formation of the amethyst. The intersection of the isochore representing the early fluid inclusions with solidus temperature of the host granite indicates initial quartz formation at ~600°C and 1.0–1.5 kbar. Intermediate quartz formation, associated with the high-salinity inclusions, occurred at somewhat lower temperatures (400°C) and pressures of ~1 kbar. The amethystine quartz formed from H2O–CO2–NaCl fluids at temperatures between 280 and 400°C, and pressures of ~1 kbar. Based on the texture and mineralogy of host minerals and on the fluid inclusion characteristics, the euhedral quartz began growing at near solidus conditions of the granite and the pressure did not vary significantly until the end of crystallization of amethystine quartz crystals in cavities. Early quartz in cavities formed from moderately saline fluids that either exsolved from or were in equilibrium with the granite, whereas the amethystine quartz apparently grew from fluids of at least partial sedimentary origin. The granite crystallized at considerable depth under relatively low water pressures probably in the root zones of porphyry-type systems. Hydrothermal activities, fluid compositions and erosion factors combined to provide favourable conditions for the formation of the Eonyang amethyst deposit and its presence near the Earth's surface today.

The amethyst geodes observed in the tholeiitic basaltic flow from the Triz quarry at Ametista do Sul (Rio Grande do Sul, Brazil) show particular wall-layering infillings with, from the outside inwards, celadonite, chalcedony, fine-grained quartz and large-sized amethyst crystals. The primary fluid inclusions analysed in the amethyst crystals yield a 152 to 238 °C crystallization temperature range. The amethyst geodes are always located in the massive, fracture-free, basaltic part of the lava flows and surrounded by a limited wall-lining alteration halo, the thickness of which depends on the geode radius. The geochemical balances calculated from fresh to altered basalt in the geode environment indicate that the amount of SiO2 released from the alteration halo is always sufficient to produce the siliceous rims in the geodes without requirement for extraneous silica supply. The results point to a volcanic origin for the amethyst geode infillings through basalt ‘autoalteration’ by authigenic high temperature hydrothermal fluids. These fluids can originate from (1) unmixing of volatile coumpounds from melt through pressure release, (2) cooling to supercritical hydrous fluids with ‘autoalteration’ of the surrounding basalt and (3) migration of the residual hot fluids from the basalt to the geodic cavities through the pressure gradient between the geodic cavity and the rock.

Size distribution data obtained from detailed field study of bubbles and amethyst-geodes in the basaltic lava flows of the Serra Geral Formation (Ametista do Sul, Rio Grande do Sul, Brazil) are used in cooling and vesiculation models to infer the origin and the formation processes of the geodic cavities. Coupled field observations and modelling results emphasize that (1) the formation of geodes in the studied lava flow can be explained, qualitatively and quantitatively, by the exsolution of magmatic gas from the supersaturated melt with no need for external surface water supply; (2) the vertically elongated habits of the geodes result from higher cooling rate of the magma in contact with the accumulating bubbles; and (3) the abnormal metre-sized geodes with their branching habits result not only from the diffusive/decompressive bubble growth but also from the coalescence of inwards-progressing tubular cavities.