Agates (Background Briefing)

  1. What is an Agate?

  There is no easy one line descriptive answer to
  fill this space. Simply, in general, the term is
  applied to any hard quartz (SiO2) based stone
  that exhibits bands of differing colour,
  tonal variation, or other unusual patterns.
  Just to add to the confusion for the
  beginner, there are many sedimentary
  rocks and metamorphic marbles that
  have similar features. However,
  the hardness of the stone is perhaps
  the key phrase that applies
  to Agate! Hopefully, the
  photographs featured in
  these articles about
  Agates will help.

                                                                                                Most of us                                                                                                 have                                                                            encountered these types of                                                                            Agate slices either as                                                                            windchimes, pendants set                                                                            with silver/gold like rims,                                                                              or perhaps bigger                                                                              specimens mounted as a                                                                              decorative table plaque                                                                              or drilled and fitted with                                                                              a clock movement (similar                                                           to some dinner plates and records).                                                           Usually, they are on sale in Mystic                                                           shops and stalls or at Tourist resorts.                                                           To the touch they feel like glass and                                                           the colours look artificial, which indeed                                           they are! These are dyed Agate slices and started                                           life as a rather boring banding of shades of grey.                                           Some enterprising spirit, realizing that rocks are                                           porous, learnt the trick of enhancing them with                                           colour (at least the first few microns of the                                 surface). No matter what you think of dyed slices, in                                 trying to understand the geology of Agates it is worth                                 looking at the banding and attempting to model the                                 growth of these stones. You may even find a feature                                 called the tube of evulsion, if you're lucky.

                      2. Where do you find Agates?

                     Begin by imagining fresh volcanic lava cooling as gas bubbles                      attempt to rise through the sticky mass! The bubbles become                      trapped leaving an empty pocket enclosed by lava. Later, fluids                      rich in dissolved minerals percolate through the lava and into the                      voids where the saturate solutions of chemicals precipitate on the walls of the pocket making it smaller and smaller until the void becomes completely filled with the agate. (Something similar to the way a kettle furs up with carbonate deposits from the tap water). The answer
to finding agates is where you find the right source
rocks. In short, experience. (Usually of others and that's
where joining a club helps!).

RC 09/09/2001