Foil (metal)
A foil is a very thin sheet of metal, usually made by hammering or rolling. Foils are most easily made with malleable metals, such as aluminium, copper, tin, and gold. Foils usually bend under their own weight and can be torn easily. The more malleable a metal, the thinner foil can be made with it. For example, aluminium foil is usually about 1/1000 inch (0.03 mm), whereas gold (more malleable than aluminium) can be made into foil only a few atoms thick, called gold leaf. Extremely thin foil is called metal leaf. Leaf tears very easily and must be picked up with special brushes.
Zari
In ancient times, when precious metals were cheaply and easily available, only real zari threads were produced. Due to industrial revolution and invention of electroplating process, imitation techniques came into existence to cut the cost of precious metals. As copper is the most malleable and ductile metal after gold and silver, silver electroplated copper wire replaced pure silver. Various modern colours and chemicals are used to create/impart a golden hue instead of pure gold. The precious metals and copper too became dearer due to huge demand in various modern industries. Thus, a cheap and durable alternative was invented with non-tarnishing properties. Metallic zari came into vogue replacing traditional metals like gold, silver & copper. This non-genuine modern zari is light in weight & more durable than earlier editions. Also, it has the sought after properties of resistance to tarnishing and knotting.