金子 means "gold" and refers to the precious metal used in jewelry, currency, and trade. The suffix 子 gives the word a colloquial, everyday feel compared to the more formal 黄金. It is commonly used when talking about gold as a material or commodity.
这条项链是纯金子做的。
zhè tiáo xiàngliàn shì chún jīnzi zuò de.
This necklace is made of pure gold.
古代人用金子来交换货物。
gǔdài rén yòng jīnzi lái jiāohuàn huòwù.
Ancient people used gold to exchange for goods.
金子的价格最近涨了不少。
jīnzi de jiàgé zuìjìn zhǎngle bù shǎo.
The price of gold has risen quite a lot recently.