Bob,
The problems of specimens loosing their color and altering in other ways have been commented on by collectors and curators for more than 100 years. In his Popular Guide to Minerals (1912), Louis Gratacap wrote unhappily of the deterioration of many specimens in the Clarence Bement collection at the American Museum of Natural History: "I have noticed that specimens of even vanadinite, descloizite, and rhodonite, lose something of their initial brilliancy and intensity under the scourge of that actinic bombardment to which they become exposed in our halls. The fluorites, pink quartzes, even the delicate greens of some spodumene, the faintly blushing calcites and the rhodochrosites, also sensibly succumb to these exposures, while it is a matter of common annoyance to find that the realgars, cerargyrites, proustites, cuprites, crocoite, and sulphurs go through changes that slowly alter their substance, texture and appearance. Stibnite loses its splendent surface, cut topazes pale, and some colors in barite slowly vanish. Mercury minerals are altered, cerargyrite changes and andorite tarnishes.
Your problems seem to be related to only dolomite and blue chalcedony. It has been my observation that colors, especially pale/pastel types of colors frequently are light sensitive and I think the only way to preserve them long term is to keep them in the dark in cool dry places. Have you ever put any of your blue chalcedony out in the direct sunlight for a while to see how they react? I would wager that the blue would pretty quickly "bleach out" and that soaking it in water and or oil will not restore it to its original color. I would speculate that its loss of color is probably due to the destruction of color centers in the chalcedony by UV light and the loss of water from the structure of the agate. This should be something you should be able to test easily. Most chalcedony is somewhat porous and that is why it has been dyed for years to make it look prettier. Taking one of your nice blue chalcedony's fresh from the ground and soaking it in oil will probably make it look substantially bluer. I know that is the case with the blue agate we have just received from Malawi.
Dolomite is the other mineral you have in your geodes that sometimes seems to give you problems. Generally I think of it as being more stable than calcite. I know it doesn't react as readily with hydrochloric acid as calcite. If you are getting various results with dolomite from different localities, that would suggest that either you are subjecting them to different storage environments or that the composition of the dolomite is changing from one locality to the next which is probably more likely the case. The only way to run this down would probably be to do careful analytical work on those dolomites to see if there were chemical and or trace element differences in them. Eventually the advent of fast and cheap analytically equipment may make it practical to do that. I have heard that much less expensive mass spectrometers are on their way and those may for example make it possible for us to at long last determine what localities various mineral specimens come from once analysis from know specimens are determined. It could be done now, probably but the cost of doing it could not be justified compared to the knowledge gained.
I would suspect that your best bet is going to be to display only those only those specimens your experience has found to be stable and keep the rest of them in a dark cool place. I suspect that the polluted atmosphere we have in our cities plays a more important part than we know in the gradual degradation of our minerals than we generally suspect. I have kept most of my specimens from day one in flats in as fry and a cool place as I can manage and they seem to retain their "freshness" better than those that have been kept on display, but as you noted, it depends on the specimen.
Rock Currier
Crystals not pistols.
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