Translucent clay can be used for a variety of special effects. You can mix in small amounts of colored clay to produce a watercolor effect. Translucent clay by itself has a look something like alabaster. You can use a thin layer of translucent clay over a strongly-colored pattern to create a milky surface. And translucent clays are often used in combination with colors to produce faux stones like ivory, jade, and amber.
You can use translucent clays for anything you use colored clays for; conditioning, mixing and marbling, canework, or anything else. However, there are a few overall differences. Translucent clays are often softer and stickier when conditioned than the same brand of colored clay. Also, many translucent clays require a lower firing temperature to avoid darkening; see the instructions on the package. (If you're using colored and translucent clay in the same piece, fire at the lower temperature.)
Most clay manufacturers offer one or more translucent clays.
Sculpey translucent #510 is one of the most transparent of the clays, very soft, and tends to pick up a yellowish tinge when fired longer than the minimum time.
Fimo offers two translucent clays. Fimo art translucent #00, which is only sold in 12-ounce blocks, is preferred for making faux stones because the fired clay develops plaques, tiny imperfections that enhance the clay's resemblance to natural stone. Fimo transparent/opaque #01 is one of the least transparent of the translucent clays, and unlike art translucent, it does not readily change color at normal firing temperatures of 250-275° Fahrenheit.
The translucent clays are not truly transparent, but you can increase their transparency level:
As of now, there are no truly transparent polymer clays (and given the limitations of polymer chemistry, there may never be such clays). You can obtain a near-transparent effect by putting a very thin layer of translucent clay over an object, firing, then wet-sanding and buffing it, although there will be some cloudiness.
You can also use glass marbles or stones as a transparent element of your design along with the clay. Check out the article in the July/August 1996 issue of Jewelry Crafts magazine on using glass marbles with tiny polymer-clay designs to create beads.
Transparent clays can develop tiny flaws while firing. The effect is called "plaquing". Fimo art translucent #00 in particular shows this effect. Some artists seek out this effect, because it can be used in faux stones to make the pieces look more like natural stone.
You can reduce the plaquing by using a different translucent clay - for example, Fimo transparent/opaque #01 - or by placing your pieces in a cold oven and then turning it on for firing, then letting the pieces cool down in the oven after firing. On the other hand, if you want the plaquing effect you can enhance it by putting your pieces into a preheated oven and removing them as soon as firing is complete. Wet-sanding and buffing will also make the plaquing more visible.