Hill & Forti write, p.250, in Cave Minerals of the World, 2nd edition about macro- and monocrystalline speleothems, the description of conditions for their growth concludes "such conditions often exist in closed caves (i.e. those which have no natural entrances or those which have experienced recent entrance collapse).
A good example where opening a cave has meant the loss of such lovely reflective macrocrystalline "facets" would be the Mendip showcave of Gough's Cave - Diamond chamber (aptly named) was originally heralded as a superb chamber, the description of which quite clearly related to macrocrystalline facets. This chamber no longer exhibits this - perhaps as a result of the opening of the cave but maybe other factors are involved.
Anyone not familiar with macrocrystalline "facets" speleothems can see film of this in Third Eye Film's "A rock and a hard place" where the "discoverers" enter a chamber beyond Tor Hall, remarking "I've never seen flowstone like that before" & "Twinkly!".
It would be good to get some feedback on whether air sealing a previously closed cave would help conserve such formations; I have no expertise in this area - Hill, Forti, Maltsev et al spring to mind as authorities who could shed more light on this.
P.S. I'm getting fed up of the post-a-reply feature in this software which has a bug in it whereby it proves necessary to retype the whole reply again; this has happened to me too many times recently and has occasionally meant that I've typed something in, lost it and then simply not bothered to retype it again; in this instance I've decided to plug on and redo the work. Have others experienced this problem? Also, if I use the "back" feature in Microsoft Internet Explorer after selecting a topic from the main forum index, I am taken back to the old greenish forum index where the conservation topic isn't even listed so that's another bug, presumably. Other than that, everything seems to work fine.