Fluorine

Now, respectfully, I must take up a little bit of a qualm with your claim in fluorine...You mention that "There is no transparent container that will hold it." Granted that is true if you're talking a "forever" time scale, but I strongly believe on a "realistic" scale (a few decades) it can be done...albeit with some difficulty and great time placed into it. The way best to do it is first to get yourself a pure, single-crystal quartz tube...Now that means one with an extremely high amount of surface Si-O-Si bonds and VERY few Si-OH endcaps. The best way to do this is to take the inside portion of the quartz tube and silylate it. Then anneal it at the highest possible temperature that your annealing oven can stand...This will drive off essentially ALLLLLLL the Si-OH end caps. Because remember, the real killer in fluorine gas for Si-O's is not the fluorine, but the OH's and their ability to start a chain reaction with small amounts of HF in the fluorine gas. So, the first thing you need to do is get rid of the Si-OH's which that should take care of as best as possible. Now, being absolutely certain that your quartz tube is flamed and ultra-dry, there's another step...There was a fluorocarbon grease that DuPont made many years ago that was ultra-high-purity completely fluorinated, medium-high mol.weight fluorocarbon grease (like a lower-molecular weight teflon)...Take that stuff and literally melt it into the tube...It's clear and translucent and won't affect the optics after the next step...So then take a high temperature vacuum oven and turn the tube upside down and melt the grease back out...What this does is leave a verrrrry thin, essentially invisible layer of fluorocarbon grease layer on the inside of the tube. This layer acts as a secondary "buffer" layer to the quartz. So IF there are any Si-OH's left on your quartz, they are difficult to get at by the fluorine gas because the fluorine gas has a difficult time penetrating the grease...This step will add years to your fluorine gas display. Then the more difficult thing to do is to make sure the quartz tube has a high-purity teflon screw-top stopper to it so that it can seal ultra-tightly. (again, pure fluorine gas without any water/HF in it may "trade" fluorines with teflon, but you still have teflon; same goes for the grease, the grease may "trade" fluorines as we've seen in some isotopic studies, but it remains a carbon-fluorine bond).
Then you should get a sacrificial vacuum line (kind of expensive, but it'll just be fogged up after you're done though it's best to throw it away because the integrity will be damaged) and run your fluorine gas THROUGH A LIQUID NITROGEN FILLED TRAP into your evacuated quartz tube. This is the most important AND DANGEROUS step. This step is the most important because ALLLLL commercial fluorine sources have either water or HF in them. The water and HF are what will start the "chain reaction" of eating away at things. And all it takes are a few atoms of these to get it started...But the N2(l) will definitely remove ALL of them...But the fluorine gas will still have a small amount of volatility to it so as to fill your quartz tube with approximately a quarter-atmosphere of pure F2. Now if you do anything with fluorine and leave ANY HF or H2O in it, fuggetaboutit...You'll get your stuff eaten away promptly. You won't get a full atmosphere of fluorine in your quartz sample tube like I said, but it will be enough to see under the right light and circumstances. And 50 years from now those one or two atoms of HF and H2O that are left in there will eventually have done enough damage to destroy your sample tube, but I don't plan on worrying about it 50 years from now.
Sorry if that bored you...But I do say it with utmost respect...I spent 10 years fiddling and trying to perfect the best way to get a fluorine sample, and that's the best way I could get it...So, methinks there are ways to store fluorine safely in a visible specimen tube; it just takes a great amount of patience, diligence, safety-thoughts and equipment.

Fluorite Crystals, Kendall Mtn. near Silverton, San Juan County, Colorado.
This specimen was found in the steep section of tailings of an abandoned mine. These crystals fluoresce a nice vibrant powder blue or purplish blue in ultra-violet light.

Banded Fluorite, Sierra County, New Mexico
This specimen was found in the steep section of tailings of an abandoned mine. Also included with some specimens are quartz, barite, granite and/or jasper. It fluoresces a nice vibrant purplish blue in both short & long wave ultra-violet light.