Silicates: Chalcedony

Composition: SiO₂; the cryptocrystalline variety is called chalcedony, the coarsely crystalline variety is called quartz

Crystal system: Trigonal; massive, banded and colliform forms

5.8.1
stalactitic chalcedony

Properties:

• Vitreous lustre
• Variable color; white, cream, red, yellow, green, blue, black
• White streak
• Knife does not scratch (Mohs Hardness 7)
• Average density
• No cleavage
• Smooth fracture
• Not ferromagnetic
• Does not react with hydrochloric acid (HCl)

Occurrence:

Chalcedony is composed of minute quartz crystals formed by crystallisation of originally colloidal silica deposited at low temperature in veins and cavities in volcanic and sedimentary rocks or by replacement of other materials such as asbestos and fossil wood

5.8.2 stalactitic chalcedony	5.8.3 stalactitic chalcedony

5.8.4 agate 'thunder egg' Agate Creek north Qld Australia	5.8.5 jasper

Colored and banded varieties of chalcedony used for jewellery and ornaments include agate (banded), carnelian, bloodstone and jasper (reds and green), tigereye (yellow or blue) and petrified wood.