Let's clear firstly the name and than the meaning:
The Oxford Dictionary explains (as dictionary) only very short: Since 1676 in use coming from French "géode" or Latin "geodes" = Greek gaiodes = earthy. (a) A modular stone, containing a cavity usually lined with crystals or mineral matter, (b) The cavity itself; also any similar formation (since 1849)...
Having a look to e.g. Columbia Encyclopedia you
"geode (je'ad), hollow, globular rock nodule ranging in diameter from 1 to 12 in. (2.54-30.5 cm) or more. Most geodes are partly filled with mineral matter; they have a thin layer of chalcedony ("wavy" quartz) covering an inner lining of inward-project-ing crystals. These spectacular crystals, generally quartz or, less often, calcite, make geodes highly prized by collectors. Geodes are formed in a cavity such as might be found inside a fossil shell buried in sediment. At the beginning, this cavity is probably filled with a concentrated salt solution. The first step in the creation of a geode is the formation along the inner cavity wall of a layer of gelatinous silica, which will eventually be transformed into the chal-cedony layer. As the water surrounding the layer be-comes less salty, osmosis induces migration of fluids into the cavity. This results in a buildup of pressure, causing the cavity to expand until the water inside and outside are equally salty. When the silica gel dehydrates, crystallizes to form chalcedony, and cracks, mineral-bearing water enters to slowly de-posit the inward-projecting crystals. See CONCRETION.
The piece, we wish to have a look to, is belonging to the bigger ones, but it is not a geode, it is only a slice of it. It's dimenstions are 272 x 200 x 6 mm. Our Eyes are only constructed to see to a matter or through a matter. This creates two different impressions (on top of the next page). It does not say much "different expressions" - Two different worlds! - In addition to that we have the possibility to look to both the negative pictures and find again two different worlds.
After clicking you firstly find four pictures. Upper line left = on-sight, right = through-sight. Lower line left = on-sight negative, right = through-sight negative. If you click on the one or the other you see a bigger version of the one you clicked on. This shows 12 sections. You can enlarge each section by clicking on it. Below you find letters A, B, and C. If you click there, you will see a composite of 3 vertical sections to having a better lookover. The loading time of this is three times longer than if you click on one section only.
If you wish to go back to the complete sight, you use the return of your browser, if you click again you enter a string as follows (a) section always the same, version string: positive on-sight > negative on sight > positive through-sight > negative through-sight > complete item.
By clicking on the composite of 3 vertical units you will return straight to the complete item.
If you like to having a look..... Please start now - click here!
Hamburg, January 2004