A gem is a cut and polished piece of mineral substance, either precious or semi-precious in quality. These mineral pieces are called stones , a term used especially by jewellers and one which can be applied to the mineral pieces in either their original or cut and polished form. Diamonds, emeralds and rubies are precious stones ; jade, amethyst and garnet are semi-precious. Jewel can be used interchangeably with gem , although it more often designates a precious than a semi-precious stone . Jewels are cut and polished and usually set into a brooch, necklace or other article of personal adornment. The article itself can be referred to as a jewel , but is probably more often described as jewellery . Jewellery is an inclusive word that embraces not only gems and jewels that have been set but also those articles of small worth known as costume jewellery : rings, tiepins, etc., manufactured with inexpensive metals, plastic, or imitation stones .

Gem and jewel are both used figuratively and informally to describe a person or thing that is valuable or highly thought of. A gem is something that has intrinsic worth, a jewel something one prizes for personal as well as objective reasons.

• That Daumier is a gem of a lithograph; It’s a jewel of a restaurant, the chef’s salad is divine.

People characterized as jewels usually have done something to warrant the appellation.

• My babysitter is a jewel ; she cares for the children as if they were her own.

People who are gems may be jewels as well, but they might be so named simply because of some prominent trait in their nature, even an eccentric one: Miss Skinner is a perfect gem , the prototype of the spinster schoolmarm.

Ice and rock are American slang terms. Ice means diamonds, but is used loosely to mean valuable jewellery of any kind. A rock is a precious stonle , especially a large diamond.