It is possible to invest in gemstones, the basic idea is like precious metals where you buy something and then sit on it until you want to sell it and hope it has gone up in value. However with gemstones their value is not as well defined as with precious metals, there is no spot price for a diamond. The gemstone markets are full of ups and downs as certain types of stones become fashionable for a while before fading into obscurity once more. In general, gemstone prices have just about kept up with inflation. Part of the problem is that the market is not very liquid and trying to sell gemstones is very difficult. The valuation of a gemstone is also very subjective.
If you really want to invest in gemstones then you can either buy only certified gemstones from reputable certified dealers for a high price or you can buy small, cheap, low quality stones and hope that one of them becomes fashionable when it is time to sell them. The general rule of thumb is that you make your money when you buy the gemstones and not when you sell it. In other words, you have to buy at less than market price because you will not get more than market price when you try and sell it.
Buying gemstones from your local jeweler is the expensive way to go. The average jewelers markup is in the 100's of percent. The stone has had to travel from the miner to the cutter to a local distributor to a distributor in the country of sale to a jeweler. Each time it changes hands there is a markup. Your best bet is to find the distributors in the country of origin and purchase from them. There are a few online auction places that get their gemstones directly from the source and then sell them to the public.
One idea is to find some cheap gemstones online and then go to a local jewelers to have them set in rings or pendants. You end up with jewelry at a cheaper price than if you had bought them already made and you can sell the jewelry later if you want to. You can find cheap gemstones online at an auction or even buy packets of them on Amazon. You are not after anything expensive because its value is included in the price and every dollar extra that we pay when we buy is one less dollar we get when we sell. Any cheap gemstones you do not get set you can just store as loose gemstones. When the time comes to sell them you might find that the cheap stones of today are now fashionable and everybody wants them. For about $20 per packet it is probably worth a bit of a gamble.
If you are going to spend a bunch of money on a gemstone then get it appraised by an independent authority first. The general rule is, if you do not understand how the gemstones are graded then stay away from the graded ones.
Auction site for Gemstones.
Gemrock Auctions - www.gemrockauctions.com