A translucent gemstone or mineral is one which allows some light to pass through but is not completely transparent. It is not always immediately apparent if a mineral is translucent and it's very easy to be fooled into thinking it's opaque. Unless completely familiar with a mineral variety, it's always worth checking for translucency with the aid of a bright light as this characteristic will often make it possible to see fascinating inclusions or other visual anomalies. In some cases only particular areas or even a single patch may be translucent.
Gemstone varieties which exhibit a good degree of translucency are often faceted because these cuts emphasize the way in which they interact with light and faceting a stone also helps to enhance its colour. Most of the world's finest gemstones including diamonds, rubies and sapphires are faceted in order to maximise their beauty.