The Definition of Translucent
A translucent mineral or gemstone is one which allows some light to pass through but is not fully transparent. It may not always be immediately apparent whether a stone is translucent but hold it up towards the light and you'll see immediately whether any part is translucent.
The degree of translucence can vary considerably from one stone to the next. An example is agate which can be almost opaque to highly translucent. Using the word translucent in relation to minerals and gemstones identifies that some amount of light can pass through. An opaque material prevents any light from passing through whilst one described as transparent allows all light to pass through.
The degree of translucence in a mineral or gemstone will often deteriorate with the presence of flaws, inclusions and impurities. A gemstone is described as being translucent if light passes through but it's not possible to clearly see an image through the stone. Clear quartz is transparent because images can be clearly seen through the stone.
Transparent and translucent gemstones are usually faceted whilst those that are opaque tend to be cut as cabochons.