Geologically, Teän is the eastern end of an old upland - a series of granite tors connected by low-lying ground covered in glacial till and outwash gravels. Glacial erratics cluster on the north coast beaches, and the island has been designated a Geological Conservation Review site because of what those gravels reveal: this was the southernmost edge of the great Quaternary ice sheets that came down over Britain. The highest point, Great Hill, rises to forty metres at the eastern end. The island is roughly sixteen hectares, sandwiched between Tresco a kilometre and a half to the west and St Martin's just 300 metres to the east. At extreme low tides the channel between Teän and St Martin's almost closes; in the Bronze Age and Romano-British era, when sea levels were lower, these were not really separate islands at all.

On the western slope, near the modern landing at West Porth, there are the ruins of an early Christian chapel possibly dedicated to a saint called Theon - sometimes recorded as Theona. Sixteen graves have been excavated under the east wall of the chapel, suggesting the church was rebuilt on top of an even older cemetery, and that there was probably an earlier wooden building before the masonry one. The same valley holds field boundaries from the Romano-British period, only visible at the lowest tides. Whoever Theon was, he or she has left no biography. The graves themselves speak: people from this side of the world's edge were buried in a way that mattered to the people doing the burying, and the dead were tended for centuries before the chapel was finally abandoned.

A Parliamentary survey of 1652 reported one man living in a ruined house on Teän. By 1684 there was a proper thatched cottage between East Porth and West Porth, and it belonged to a Mr Nance, who is credited with introducing kelp burning to the Scilly Isles. Kelp burning, the process of harvesting and slow-roasting seaweed in pits to produce a crude ash rich in sodium carbonate, fed the English glass industry through the 18th and into the 19th century - a low-yield, smoky, labour-intensive trade that produced only two or three percent useful sodium carbonate by weight but employed thousands of islanders. Rights to kelp on Teän belonged to the Nance family for generations. In 1787 three men from St Martin's were fined 2s 6d each for poaching seaweed from Nance's section. By 1717 ten people lived on the island. By 1752 the antiquarian William Borlase found only ruined buildings and fields of corn. By 1919 a guidebook called it a rabbit warren. Cattle were still being grazed here in 1945.

Teän was notified as a Site of Special Scientific Interest in 1971 and the last full assessment, in September 2009, found the island in favourable condition. Among the rarities behind the dunes at East and West Porth grows the dwarf pansy, Viola kitaibeliana, one of Britain's rarest flowers and almost confined to Scilly. Saltwort, once common on every Scilly beach but now scarce, was recorded again on Teän in 2009. Orange bird's-foot, a small leguminous plant that needs short turf to survive, grows near Clodgie Point. Mammals are sparse - brown rats and house mice are all that remain; the Scilly shrew has not been recorded since 1964. In 2008 a pair of marsh harriers nested here and raised two chicks, the first breeding record in the islands for many years. The kelp pits are gone. The pansies bloom in May.