A substance of economic value that can be replaced or replenished in the same amount or less time as it takes to draw the supply down. Some renewable resources have essentially an endless supply, such as solar energy, wind energy and geothermal pressure, while other resources are considered renewable even though some time or effort must go into their renewal, such as wood, oxygen, leather and fish. Most precious metals are considered renewable as well; even though they are not naturally replaced, they can be recycled because they are not destroyed during their extraction and use.   Renewable resources have become a focal point of the environmental movement, both politically and economically. Energy obtained from renewable resources puts much less strain on the limited supply of fossil fuels (non-renewable resources). The problem with using renewable resources on a large scale is a cost problem and in most cases, more research is needed to make their use cost-effective.