[2] The island is fringed with detritus consisting of eroded calcarenite and granite boulders which a talus slope rises to a “relatively flat, plateau-like summit” at a height of 38 metres (125 feet) above sea level.

[3] Royston Island consists of a calcarenite upper layer over a “pronounced ridge of Lincoln Complex granite.”[1] Royston Island is fringed partly on its north-west and south-east ends by fringing reefs while the waters surrounding its extent drop to a depth of 20 metres (66 feet) within a distance of 1 kilometre (0.62 miles) on its westside.

The island's summit is dominated by a “low, windswept shrubland” consisting of marsh saltbush and coast daisy-bush accompanied by less abundant species such as southern sea-heath, round-leaved pigface and pointed twinleaf.

The talus slopes support shrubs of nitre-bush, native juniper, cockies tongue, African boxthorn, yellow-flowering sticky goodenia, Austral stork's bill, coastal lignum and ruby saltbush, while calcrete ledges associated with the island's upper levels are “festooned with climbing bower spinach.”[1][4] Vertebrate animals are represented by birds and reptiles.

Since 2012, the waters adjoining the shoreline of Royston Island have been within a habitat protection zone in the South Spencer Gulf Marine Park.