Answer :
This is because gold in its pure form is very malleable i.e. it can be easily modified into shapes by beating it with hammer and very dense.So if ornaments are made out of pure gold then a small force can cause the ornament to collide.So some amount of silver or copper makes gold strong and less malleable.That's why goldsmiths and jewellers add copper (often) and silver (rarely) to gold when making jewellery. Copper is melted together with gold to make an alloy, which can then be beaten, drawn or twisted into various patterns.Gold for jewellery is measured in carats. 24 carats refers to the purest (99%) gold. It's used for making gold biscuits and coins. 22 carats (91% gold) and 18 carats (75% gold) are more popular for making jewellery.