On the topic of most value per weight or volume, or "what would I choose if I had to flee the country", gemstones might still be the best choice. I recall reading that some people fleeing the Nazis through Antwerp choose gemstones for this reason. Even now they have some advantages. You could glue two dozen high quality 2-karat diamonds in the soles of your dirtiest shoes and waltz through any X-ray, scanner, or other search without a fear, and they could be worth hundreds of $ each after you were through. Granted you should assume a 75% loss of value or something, but in this case you would be happy to have any value. Gemstones still seem to come out ahead of gold or paper money for this use.
I've lived in countries in which economies and governments collapsed. In my experience, if a person thinks something like this is likely enough to want to prepare for it, having gold coins or gemstones is at best inefficient and at worst dangerous. Better options include:
--Have an education that gives you useful skills. If you have to flee your own country and go somewhere else, a good diesel mechanic can find a job in minutes anywhere in the world. A medical doctor, CPA, or engineer will probably get a job eventually, but will probably have to get local certifications first. This can take a long time.
--Don't flaunt your wealth before things collapse. If you're living large, you make yourself a target.
--Have money and property overseas before things go bad. Set up a foreign bank account and deposits in a currency that gives you confidence. Better yet, invest in income-producing property in a foreign country (preferably one with an investor visa program) in which you have confidence and keep the earnings in that country.
--When fleeing a country, high value items don't give you power, they make you a target. If you are planning to bribe a border guard, you offer him a large pile of rapidly devaluing cash that will buy a "real" hundred dollars of goods and he/she will probably be happy. If you offer a one ounce gold coin, not only will you not get change, you'll probably get pulled into secondary to search for all the other gold coins they hope you're carrying. And don't assume that you will be able to "waltz through" a border with millions of dollars of commodities on your person. When things are going bad, corrupt officials get very motivated to identify people who are leaving and to find their money. In the movies, there's honor among thieves.
In the real world, the people who will sell you two dozen 2-carat diamonds will also happily rat you out for a percentage of the take. And when you do get caught violating currency export laws, you may well lose everything, including your chance to escape.
The real key is getting out early. The earliest refugees are almost always the most successful because they leave early while they can do so with a plan and with assets. Once things go really bad, very few people get out with much.
There are similar problems with trying to use gold, gemstones, or collectibles in a collapsing economy. When you want to buy a few gallons of gas for your car, a loaf of bread, or a pack of toilet paper, you can't pay for it in gold coins. The guy at the gas station doesn't know what a gold coin or a diamond or a Rolex is worth, can't be sure it isn't fake, isn't going to take a small fraction of the coin or diamond, and can't make change. And if he does believe that your gold coin/diamond/Rolex is real, he's going to be tempted to call someone who will just rob you and give him a finder's fee.
FWIW, I have made no preparations. The US has made it through almost 250 years of challenges without collapsing and I believe it will continue through at least my lifetime. But if you want a contingency plan for a collapse, there are far better tools than gold coins and gemstones.