Abstract: Samples of agate and quartz incrustations from different parent volcanic rocks of certain world-wide localities were investigated by EPR, CL and trace element analysis. In all agate samples the following paramagnetic centres were detected: O 2 3- , E′₁, [AlO₄]⁰ [FeO₄/M⁺]⁰ and [GeO₄/M⁺]⁰. Centres of the type [TiO₄/Li⁺]⁰ and [TiO₄/H⁺]⁰, which were detected in quartz of the parent volcanics, are absent in agate. Generally, the abundance of O 2 3- centres (silicon vacancy) and E′₁ centres (oxygen vacancy) in agate is remarkably higher than in quartz. The high defect density in agates points to rapid growth of silica from a strongly supersaturated solution probably with a noncrystalline precursor.

CL microscopy reveals internal structures and zoning in agates and quartz incrustations which clearly differ from those discernible by conventional polarizing microscopy. The CL spectra of agates differ from those of quartz from crystalline rocks. At least three broad emission bands were detected in the CL spectra: a blue band of low intensity, a yellow band at about 580 nm, and an intense red band at 650 nm. The CL emission at 650 nm shows some relations to the hydroxyl or alkali content and the abundance of O 2 3- centres and E′₁ centres. The emission intensity increases during electron bombardement due to the conversion of different precursors (e.g. ≡Si-O-H, ≡Si-O-Na groups) into hole centres. Another conspicuous feature in the CL spectra of agates is the existence of a yellow emission band centred at around 580 nm. The predominance of the yellow CL emission band and the high concentration of E′₁ centres are typical for agates of acidic volcanics and are indicative of a close relationship between the two.