In 2010, Silhouette National Park was created protecting 93 percent of the landmass. The island is also an Important Bird Area and is considered by the Alliance for Zero Extinction to be an important site for the survival of Critically Endangered species. Conservation of the island is managed by the Island Conservation Society. Silhouette Island is one of the richest biodiversity hot-spots in the western Indian Ocean with many endemic and threatened plant and animal species. Among the most important is the Critically Endangered Seychelles sheath-tailed bat. Two roosting caves have been located, part of a single system of passages in a boulder field. Another roost complex was discovered in 2005 and 32 bats recorded. Most of the 75 or so endemic plants of the granitic islands of Seychelles are found on Silhouette, some of them unique to the island. One of the rarest of these is the Critically Endangered Impatiens gordonii, a white-flowered relative of the well-known garden plant Busy Lizzie, only ever recorded on Mahe and Silhouette.
The island is mountainous with five peaks over 500 m (1,600 ft) in elevation; Mont Dauban 751 m (2,464 ft), Mont-Pot-a-Eau 621 m (2,037 ft), Gratte Fesse 515 m (1,690 ft), Mont Corgat 502 m (1,647 ft) and Mont Cocos Marrons 500 m (1,600 ft). As such, it has some of the most dramatic scenery of the islands.
Though superficially similar to neighboring islands, much of Silhouette is made up of younger seynite dated from 63 million years ago. Between Point Ramasse Tout and Point Zeng Zeng lies the only surface volcanic ash in Seychelles.