Go to the auction house and go under gems. You can find tons of them being sold at under 1g. The vendor value for them is 1g. Therefore, buy them, sell it to a vendor, and bam, you make a small profit.
This does require a bit of money to start out with (usually 75s) and it is a limited supply. It can get you some good quick money though.
Just to let you know, this is not going to buy your your epic mount in a day, its not going to automatically give a tone of gold in an instant. But it will help. I just emptied about 40g on them, and sold. now i have over 50g. It can grow quickly.