Tuesday, 28 June 2011

Agate and Jasper ~ what is the difference?

Seeking the answer to this question I looked up my book, "Gems and Crystals".  

The first thing I learnt is that (most?) agates are part of the chalcedony family.  

On page 52 under the heading Chalcedony and Jasper they say this:
Both are made up of submicroscopic quartz grains - thus are varieties of quartz - and owe their bonanza of colours and patterns to included minute grains of other pigmenting minerals.  Chalcedony differs from jasper in that its tiny crystals are parallel fibres rather than sugar-like grains.  Distinguishing them requires a microscope, although typically chalcedony is banded and translucent. 
On page 154 they say:
Jasper's granular texture makes it tough and generally more opaque than chalcedony, and jasper lacks the other's banding.  While commonly red to ochre from iron oxide pigments, jasper can occur in a multitude of colours.  Some materials have mixed textures of both chalcedony and jasper juxtaposed in anywhere from millimetre- to centimetre-scale blotches.  Gemstone varieties that show either of both texture include bloodstone and chrysoprase.
 On page 155 they list the main chalcedony varieties as: 
  • Agate: all forms with parallel to concentric banding, transparent to opaque, including bull's eye agate and iris or fire agate
  • Moss agate - translucent light coloured body with black, brown or green moss-looking branches, usually dark oxides.
  • Onyx - bands are black and white
  • Sardonyx - bands are ochre to white
  • Bloodstone or heliotrope, which is plasma with red hematite and jasper spots and blotches
  • Carnelian - translucent red brown to brick red from hematite
  • Chrysoprase - translucent apple green from nickel serpentine
  • Plasma - opaque leek to dark green from various green silicate minerals
  • Prase - translucent leek green from chlorite inclusions
  • Sard - translucent light to chestnut brown from iron oxides and hydroxides.
This is a lovely book, packed with sumptuous photographs.  It's a pleasure to have and to browse through.

Full title details of the book referred to are: 
"Gems and Crystals: an illustrated guide to the history, lore and properties of gems and minerals" 
by Anna S Sofianides and George E Harlow 
Published by Parkgate books in 1997
ISBN: 1 85585 391 4

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