It's funny to me that even though the Hope Diamond, which is blue, is one of the most famous gemstones in the world, most people don't realize that diamonds come in colors other than white. In fact, some diamond colors, like yellow and champagne, are quite common -- just not in your typical engagement ring.

As with any gemstone, trace elements within a diamond or other elements present during the stone's formation will affect its color. The vast majority of colored diamonds really only exhibit a hint of color at best. The most valuable colored diamonds are the ones with the naturally vibrant colors. Yellows and browns are the most common and least expensive of the colored diamonds, while shades of red are the most rare and most expensive.

Colored diamonds are graded the same way as colorless diamonds, except for one of the four C's: color. Whereas white diamonds are valued for their lack of color, these stones are more valuable as the color gets more intense. The color grading of a fancy diamond actually has three categories: hue (the characteristic color), tone (the color's relative lightness or darkness), and saturation (the strength or weakness of the color). The Gemological Institute of America's grading system is the worldwide standard.

When shopping for a colored diamond, bring some knowledge to the table to avoid getting ripped off. A naturally vivid colored diamond can be extremely valuable, but these colors can also be manipulated in a lab, for an equally beautiful diamond, but slightly less valuable because the colors are not natural to the stone.

Diamond color can be irreversibly changed through a process called irradiation. There is no completely accurate way to predict what color change will occur with irradiation, and once it is done it cannot be reversed. Most irradiated diamonds started off as some undesirable shade of brown, and depending on the trace elements within the stone, they can change to any color in the spectrum. An honest jeweler will give you an honest answer about whether or not a stone has been color treated. Gemological labs can test stones to determine if their color is natural or treated, and if your colored diamond is certified, it will have this information. A laboratory cannot always tell if the color is natural, in which case they will disclose that as well. Only pay the top premium prices for stones certified as naturally colored. In most cases, these stones will be quite a bit more expensive than an equal quality colorless stone, because the truly vivid colors are very rare, even with irradiation treatment.

I would strongly advise against purchasing a colored diamond online. Color is difficult to catch accurately in a photograph, and photographs are also too easily manipulated. When I worked in the jewelry industry, most of the major scams I saw involved false representation of colored diamonds by online vendors. If you're considering buying one of these gorgeous stones, view it in person and make sure you get a certificate from a reputable gemology lab like the GIA before you make the purchase.