Not too long ago, the vast majority of outdoor furniture was all made of teak. Teak has proved to be an ideal lumber species for outdoor furniture, due to its high durability, ease of working with machine tools, its low coefficient of radial and tangential expansion (which reduces potential problems in twisting, or swelling when wet), it's attractive colour and good working properties. But such widespread use of teak, including for marine decking, has come at a considerable environmental cost. The once great forests of South East Asia have been decimated to the extent that legally logged teak is now only available in very limited quantities, which is reflected in its high price. But unfortunately, illegal logging is still rampant and in many areas of South East Asia, ensuring its continuing supply to the furniture trade.
Plantations of teak have now been established in a number of countries including Costa Rica for example (presumably planted in most cases where the great forests once existed). But this plantation teak is harvested at a much younger age that has traditionally been the case, so caution should be taken in assuming that the properties of such younger lumber (including durability) will be the same as old growth teak.
Many responsible furniture manufacturers will now only used teak which has been certified by such independent bodies as the Forest Stewardship Council. If sustainability of forests and sound forestry practices are an important consideration in your choice of furniture, then look for the FSC logo or ask your retailer to provide documentation to show from where the lumber was sourced. Note however that even if the tag accompanying any wood furniture claims that the wood is sourced from sustainable forests, unfortunately there is no firm guarantee that this is true. You may be able to make some verification by checking the FSC website, which indicates which companies and even manufacturers have been given official accreditation by the FSC.
In recent years there has been a proliferation of different lumber species used in outdoor furniture as more manufacturing plants are set up in places such as Vietnam, Thailand, Indonesia, Brazil, in particular. Unfortunately, a lot of furniture is now produced with price as the overriding consideration, with scant consideration being given to the durability or in service performance of the furniture. And all too often these days the name of the actual wood species in not even displayed