In general collecting loose gemstones is not a very good investment. Most individual collectors will be lucky to sell their gemstones for 50% of what they paid for them. Most gemstone dealers do not buy from private collectors because they have spent years gathering sources where they buy at below wholesale prices. So when it’s time to sell your collection you are left with places like EBay or Craigslist and will be competing with sellers who have built up loyal customers and have spent a great deal of time working on their reputation.
There are a few exceptions though. If you are able to buy your gemstones when a new find is discovered you will do very well in the coming years. Tanzanite is a good example of this. When this material was first discovered the prices were ridiculously low but as the mines started producing less the prices went through the roof. New discoveries happen every couple of years to some extent but the trick is knowing when they occur and buying early in the life cycle of the mine. Keeping up with news in the gemstone trade organizations is one way to do this. Another exception is extremely high-end stones such as colored diamonds, emeralds, rubies, and sapphires that are untreated and large. These ultra rare examples of gemstones often go up in value and you would be able to sell them at one of the famous auction houses like Sotheby’s. Make sure you have very good documentation when you buy these types of gems though.
Gems are the most portable source of wealth and if you live in an unstable part of the world gemstones can be an important way to travel with some of your wealth in case you need to escape for political reasons.