Danefaith: Thank you so much! "build on this lot" works *perfectly* for my main problem! I can tweak away at the building and the gems stay put! Actually, my Sim didn't even need to visit the site for me to do so, I just had to be in play (not in edit town). I should also clarify that I can enter edit town and switch households and so forth without disturbing the museum contents, its just the moment I enter build mode with that one property through there that the gems disappear. I think I confused you on that point.
Regarding the saving a copy thing, I'll respond to Anushka: Your gem collection is awesome and goodness knows, I may resort yet to downloading it to round out my museum if I give up trying to get all those spire cuts.
Forgive me for being so lazy and not just downloading it to see what you've done, but my time is short tonight -- Are you saying that the lot is not laid out with gems like the picture in the guide, rather it is empty but for a treasure chest containing all the gems? That would make sense after what you have said. I can't see how you would have managed sharing the lot as screen-captured after what you have said and I have experienced. Danefaith has solved my editing problem, but I will have to continue exploring the sharing issue.
I can save a copy of the whole town, but that serves me no better than my external harddrive backup and if I just want to load the one building, doesn't work.
I can save a copy of the building to the library (what I was hoping for) but it sends all the gems back to their original owners. Apparently "donation" means "temporary loan". I was sort of hoping that when a Sim donated something like a gem, it detached from their inventory/person and became a stand-alone object in the lot file. Clearly this is not the case.
Using the treasure chest as you have done seems to allow the objects to detach from their original owners, but the downside is you have to place everything yourself and can only use a collection from a single owner and plan well ahead with your project. I deliberately wanted to build the museum *first* and have multiple Sims visit and donate on site, as things are found, to create a true neighborhood repository (with each collector's info attached to the donations). The backlash of this is that it is truly a neighborhood collection and therefore, can't be pulled out of the running game to another neighborhood or exchange without being lost. So, if I share the museum, it will have to be as an empty building and each person who uses it has to build their own collection. And my only backup will be my actual backup of the whole town, not just the lot.
Thanks to both of you for your help!