Volcanic Amethyst Geodes
They are hollow, crystal-lined globular rock cavities found in volcanic rocks. Volcanic lavas, eg. basalts, often have round or almond shaped gas holes. Ground waters carrying dissolved silica seep into the holes, lining the cavities with a bed of crystals pointing inwards.
Size: 40 cm
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Rock Crystal – colorless variety of pure Quartz
Crystals with typically six-sided prisms terminating with six-sided pyramids at each end.
Size: 15 cm
Rock Crystal
Tectosilicates
SiO2
Brazil
Size: 23 cm
Amethyst – a purple gemstone variety of Quartz
Colored by trace elements of ferric iron.
Size: 18 cm
Amethyst
Tectosilicates
SiO2
Brasilia, Brazil & California, USA
Size: (Left) 12 cm / (Right) 5mm (Gem)
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Rose Quartz – pink variety of Quartz
Colored by trace amounts of titanium or manganese.
Size: 20 cm
Rose Quartz
Tectosilicates
SiO2
Minas Gerais, Brazil & USA
Size: (Left) 4mm (gem) / (Right) 12 cm
Milky Quartz – cloudy, white variety of Quartz
Colored by minute inclusions of gas and/or liquid trapped during crystallization.
Size: 18 cm
Milky Quartz
Tectosilicates
SiO2
Minas Gerais, Brazil
Size: 25 cm
Smoky quartz – black or brown variety of Quartz
The color is caused by exposure to natural (or artificial) radioactivity.
Size: (Left) 3 cm / (Right) 5 cm
Smoky Quartz
Tectosilicates
SiO2
North Carolina, USA & Ontario, Canada
Size: 20 cm
Ferruginous Quartz
Tectosilicates
SiO2
Minas Gerais, Brazil
Size: 18cm
Citrine – a rare pale yellow variety of quartz
Colored by ferric impurities.
Size: (Left) 10cm
Citrine
Tectosilicates
SiO2
Brazil
Size: 15 cm
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Citrine – Yellow-brown citrine is in most cases heat-treated amethyst. On exposure to heat of 500°C, some purple amethyst crystals change their color to yellow-brown.
Size: 12 cm