Gemstones or Precious and Semi-Precious Stones +

(the four gemstones which are most valuable are diamonds, sapphires, rubies, and emeralds; and anyone would be impressed with a gift of a diamond, a sapphire, an emerald, or a ruby piece of jewelry)

diamond (s) (noun), diamonds(pl)
This is the hardest and most lustrous gem because it retains its brilliance and polish for many generations: Diamonds are a form of carbon that has been crystallized under great pressure and very high temperatures.

The two major sources for diamonds are South Africa and Australia.

"In 1695, two scientists obtained a diamond from a rich patron and heated it by using a lens to focus light on it. The diamond disappeared. Diamond is made of carbon, and it burns just as coal will when it is heated strongly enough."
—Isaac Asimov's Book of Facts by Isaac Asimov;
Bell Publishing Company; New York; 1981; page 424.
emerald (s) (noun), emeralds (pl)s,
A brittle gem which has a tendency to chip and is rarely flawless: The rich green color of emeralds are caused by the presence of chromium, a hard white metal.

In ancient times, powdered emeralds were believed to cure fever and the plague.

Emeralds are formed from a combination of three main minerals: silicon (the chief constituent of sand and quartz), aluminum, and beryllium.

Colombia is the source of the finest emeralds.

ruby (s) (noun), rubies (pl)
One of the rarest gems, especially because of the pigeon-blood red color which comes from chromium.

In the past, rubies were thought to promote good health and to keep people looking young. In addition, they were said to guard fruit trees from disease.

Some of the world's highest-quality rubies are quarried (surface excavation for extracting precious stones) from gravels in Burma.

sapphire (s) (noun), sapphires (pl)
The hardest stone after the diamond: Sapphires are normally blue, but they also exist in purple, green, white, pink, gold, and orange.

Supposedly, they have mystical qualities that include the promotion of peace and the purification of the mind.

Sapphires are formed from corundum which is a variety of aluminum oxide. Sri Lanka is a well known source of sapphires of all colors.