Investing in gems is not an overnight success. You really need to understand the market for gemstones to know what money can be made. Whilst they are easy to buy, they are not always easy to sell. White diamonds will give you a low return, as traders work on very small margins, coloured diamonds and some gemstones, normally very fine, will offer you a better return, provided you can sell them and only over a period of time.
I would be very cautious about going to a broker, because they tend to take very high margins, and you will need to wait quite a while before your stone value floats above that, before you see a return.
Walk the jewellery areas in London or Birmingham and just ask for prices, or look online, you'll see how much they vary, without you even recognising a difference in quality.