Silhouette lies 20 km north of Mahé and at 1,995ha it is the third largest island of the granitic group and the fifth largest in Seychelles as a whole, yet the human impact is much less significant than elsewhere. Mount Dauban rises to 740 metres and is the second highest summit in Seychelles. Though superficially similar to the other granitic islands, much of Silhouette (and North Island) is made up of younger rock called syenite and dated from 63 million years ago, compared to 750 million years for the other granitic islands. Silhouette may have been born in rapid and dramatic style, in a manner similar to the famous eruption of Mount St Helens that took place in western United States in 1980. It is probable that the crater lies southeast of La Passe, now almost entirely eroded away and submerged.
Silhouette supports an exceptional biodiversity, including endemic birds, amphibians, reptiles, invertebrates, bats and plants. The whole of Silhouette has been classified as an Important Bird Area, mainly because of the population of 10-20 pairs of Seychelles kestrel. Other land birds include three more endemics, Seychelles blue pigeon, Seychelles bulbul and Seychelles sunbird. About one-third of the almost 200 species of angiosperm recorded are Seychelles endemics including several endemic to Silhouette. The sheath-tailed bat has its last stronghold on Silhouette. The Seychelles fruit bat is also present in large numbers. It hosts a significant hawksbill nesting population. In 1987 the waters around Silhouette were declared a Marine National Park. In 2008, Silhouette Foundation was formed bringing together Island Conservation Society, Islands Development Company, Labriz Hotel and others to plan conservation projects on this spectacular island. In 2010, much of the island was declared a National Park and in 2011, Silhouette Island Conservation Centre was opened.