The most important thing to remember when you purchase diamonds is that they should be transparent. According to the GIA, transparency is defined as "the degree to which a material transmits light without appreciable scattering, so that objects beyond are entirely visible." In other words, the transparency of a diamond refers to how clear, hazy, cloudy, or opaque the stone is.
In order to judge a diamond for transparency, you should first look at the diamond from the sides and check if it is clear like crystal glass or purified water. When you're looking at the diamond face-up, there should be a strong contrast between the dark and light areas. You should not be able to see through the diamond. If that is the case, the diamond is not well-cut.
As with judging a diamond for its clarity, it's also important to observe a clean diamond under the following conditions: fluorescent light, incandescent light, sunlight and away from the light. It's important to note that you should observe the diamond away from white walls or objects since these can reflect into the diamond and alter its true transparency. It also helps to use a transparent diamond sample to compare your diamonds to.