I'm not sure which messages you're referring too. Diamonds have industrial uses, but they don't use gem-quality ones for that (and gold/silver have industrial uses too).

As a medium of exchange, gemstones are less convenient because they are less standard. The value of a diamond depends on its clarity, colour and cut as well as its weight, and you really need an expert to evaluate those. The value of gold - even gold jewellery - only depends on the weight of (pure) gold in it.

I harken back to the days of Mountain men and the Hudson's bay company and Rendevous.

Barter seems to be the fallback market, with practical goods being the more common exchange mecium. Precious metal minted into coin of the realm still seems to be a viable currency if one must travel and still do business. Ammunition might be suitable for a time, but it's purpose predisposes it to eventual consumption. Other items being more awkward to ferry about, it seems inevitable that some currency, not easily manipulated, fairly securable, durable, and easily recognizable, would have to manifest sooner or later.

I envision that once the initial shock of crisis is over, people will gather to exchange what they have in abundance with what they need or are want for. If folks cannot find the items they desire at one location, they are bound to travel, and will not be willing to encumber much. For this reason, I suspect a sachet of gems and coins, as in old times, will take the place of sacks of grain and sides of beef. Money is just too convenient an exchange medium, but people will not accept notes after this.

Gold and silver cannot be counterfeited. An ounce of either can be proofed readily enough, and once a minted coin receives a proof mark from a reputable local agent, it should exchange fairly routinely. In fact, for this reason junk silver is a good investment pre-collapse. It is already proofed satisfactorily that anyone can determine it's relative value. It is in portions that allow for easy whole division for market. I can easily imagine that in short order you will find traveling vendors peddling goods by coach such that he can deliver to a neighborhood things a man afoot would have trouble retrieving from any distance. While he may find barter items suitable to stock (produce from a home garden, tools, trinkets, and so on), he will also be quite interested in exchanging for common and highly fungible resources.

It may not happen right away, but depending on how bad things get and how long they stay that way, the practicality of secure currency seems inevitable. There may be a time when only material goods will trade fairly. But sooner or later, some standard must be recognized and adopted.

My father used to do work for a Jewish family that came over from Germany right after WWII. Supposedly, before things started going south for their people, they got wind of it and buried their valuables. Mostly diamonds and jewelry.

Once they got liberated from the camps, the remaining family members dug up what was buried and used that come over to the USA and start a building development company. My father can remember their father walking around at the end of the work day (with the numbers from the camp still tattooed on his hand), picking up bent nails, scrap wood, broken pieces of pipe, whatever was left on the site. A quiet unassuming man who didn't waste a single thing.

He watched their wealth grow and grow and today I know one of the sons from that family is worth around a billion dollars and is part owner of a professional football team.

Seeing that had such an impact on my father that he made sure I knew that story by heart.

Not only can you overcome dire situations if you stick together as a family..... but, if you work hard and try to waste as little as possible it might just work out for ya.

Plus, if you do end up with a little extra money here and there, it might not be a bad idea to have a little bag of diamonds and gold hidden away.

Actually, fake precious metals a already a bit of a problem. http://blog.jewelrywarehouse.com/2012/06/19/tuesday-jewelry-tips/
Nonmagnetic, and with the correct density, hardness, and a thin plating of the real stuff is going to be beyond most peoples skills to detect. There will never be a shortage of greedy sociopaths.

As far as a barter system: in the event of an economic collapse, skills, services, and odd goods will probably be the "coin of the realm".

Yes, so the ability to positively test a coin for metal content will be of some significance. For the average joe, there'd be no realistic way to make such a determination on raw metal, so a proof mark would be about as good as they could hope for.

Not too practical to travel 30 miles by foot to do work for half a dozen different people over the course of a week or more, and get paid in chickens, pigs, sacks of grain, or cords of wood. There are so many problems with that scenario, and besides, that's not how most of the crops, herds and flocks would be disbursed anyways. Far more practical to accumulate credit so that when the produce/livestock do come to market you can acquire as much as you need at an optimum exchange rate.

Direct exchange of goods and services can work. If you have a strong, well rounded community it can be viable for a while, but mobility and convenience should eventually dominate people's methods.