Pure gold is a precious metal, the least reactive of all metals, extremely malleable & does not rust or tarnish. Because pure gold is very malleable, manufacturers/jewelers generally alloy pure gold with harder metals.These harder metals have the potential to rust/tarnish.
Gold-filled jewelry is jewelry composed of a solid layer of 12k,14k or 18k gold mechanically pressure-bonded to a base metal(usually brass) & contains at least 5% gold by weight.
This technique is more durable than gold plating or vermeil. Because the gold layer bonded to the brass core is quite thick, gold-filled jewelry components have a longer life span, which is very unlikely to rub off when treated with the right care. However, under a rare set of circumstances it can still tarnish. Gold-filled jewelry is safe for most sensitive skin types.
FUN FACT
Gold is edible; Gold flakes may be added to food & drink. Edible gold leaf is a gold product that can be used to decorate food. Gold is considered "biologically inert," meaning it passes through the digestive tract without being absorbed. It is mostly used in desserts & candy making & is available as sheets or flakes. It is one of the world's priciest foods, but considering it is real gold, the sheets & flakes are relatively inexpensive. It is important to buy quality gold leaf as cheaper versions contain impurities.