Next to my own skin, her pearls. My mistress bids me wear them, warm them, until evening when I´ll brush her hair.

The opening lines of Poet Laureate, Carol Ann Duffy's poem called "Warming Her Pearls"

and one way to judge if pearls are natural, cultured or synthetic. (Read or listen to the full poem here)

JOHANNES VERMEER
MISTRESS AND MAID
(Dame en dienstbode)
c. 1666-1667
oil on canvas
The Frick Collection, New York

Pearls were an important status symbol of this age and they held for the educated Dutch art lover, a number of associations. In no other painting by Vermeer are there so many pearls. The mistress wears a pair of oversized drop earrings (artificial), a pearl necklace and strings of pearls in her hair.

When you first put them on natural and cultured pearls feel cold on the skin, they do warm up as you wear them. I can't find an explanation of why they are so cold but suspect it's to do with the poor conduction of heat. Synthetic pearls don't feel cold, unless they are made of glass. So another test is to rub the pearls on your teeth; natural and cultured pearls feel rough, synthetic ones feel smooth.
If the pearls are unstrung rub two gently together; if they offer resistance they are real or cultured pearls. If they feel smooth they are synthetic.

X-ray is the surest way to identify the type of pearl as it can differentiate between natural and cultured pearls - if you want to go to such lengths. Natural pearls are sold by weight (carat of pearls) while cultured pearls are sold by size (diameter of pearls).

Uniform natural pearls are extremely rare and very expensive.

The Baroda Pearls

In 1943, Maharaja Pratapsingh Gaekwar made headlines by marrying Sita Devi, his second wife. Referred to as “The Indian Wallis Simpson” by the Western media. Sita Devi received the jewels from the Baroda treasury including a seven strand natural pearl necklace.

The largest and most perfect pearls from the necklace were made into two large strands, consisting of sixty-eight graduated pearls, measuring approximately 9.47 to 16.04 mm, all matched in colour, lustre and shape and joined by a cushion-cut diamond Cartier clasp. In 2007 these were auctioned by Christies, along with matching brooch, earrings and ring, for the record sum of £4.5 million.

Read here in more detail.

Queen Elizabeth of Hollywood and Queen Mary1st of England
both wearing La Peregrina

La Peregrina is one of the most famous pearls in the world. Its history spans almost 500 years and it has passed from the African slave who found it at Pearl Islands in the Gulf of Panama to European Kings and Queens. Until recently, the pearl belonged to Elizabeth Taylor bought for her by Richard Burton as a Valentines gift - lucky girl!  She had it mounted on a diamond and ruby Cartier necklace. After her death it was auctioned at Christie's in New York, the bidding reached £7.1m. Watch the auction here.

Look after your pearls, they should be stored wrapped in soft cloth or in a soft-lined container to prevent them being damaged from harder items of jewellery.

Apply perfume, hair spray and other cosmetics before putting on your pearls to reduce the effects of these products on your pearls.
After wearing your pearls, wipe them with a soft damp cloth to remove any traces of cosmetic products or body oils.
The threads used to string the pearls are liable to stretch and break, be aware of this check the strands and get them restrung as necessary.