This is done because.. to make it more shiny and smooth surface.
As when light rays falls on it.. it will reflect more light and gives it a shiny apperence ... so that Jewellery piece can ce sold out easily..

Hope my answer is picked by u as best..
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For the following reasons Also - to make inexpensive jewellery for people who cannot afford pure gold ornaments. Prevents corrosion.

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Jewellery items are coated with gold and silver to make it strong.

It makes the jewel more attractive and durable.

Quality can be increased.
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