Summer on the rocks

Hardstone is the term used for any hard gem material used for carving. Hardstones in bright blues, oranges and greens produce vibrant combinations when set in yellow gold, white gold or platinum. Prominent in ancient, Art Deco and mid-century jewellery, hardstones have been used in pieces by houses such as Cartier, Van Cleef & Arpels and David Webb. Here are six of the most popular stones, illustrated with lots offered in our Christie’s Jewels Online auction, 3-13 June.

Turquoise was held dear in the ancient world by the Egyptians, the Incas and the Aztecs, and as such is one of the oldest stones known in jewellery design.

Mined in the Sinai Peninsula, Iran, Turkestan, Tibet and the United States, turquoise can range in colour from sky-blue to green depending on the quantities of iron and copper it contains. Sky-blue turquoise from Iran, referred to as ‘robin’s-egg blue’, is considered to be the most desirable.

One of the most popular hardstones throughout history, lapis lazuli was treasured for its vivid, exquisite colour by the ancient civilizations of Mesopotamia, Egypt, China, Greece and Rome, where it was prized as highly as sapphire and turquoise.

Lapis lazuli is typically cobalt blue, often with patches of pyrite (golden) and/or calcite (white). Prior to the 19th century, it was powdered to create the ultramarine pigment used by painters. In the modern day, lapis lazuli is favoured by jewellery designers such as David Webb.

The term jade is often interchanged with jadeite and nephrite without regard to the fact that they are different minerals with different properties. Nephrite, which is abundant worldwide, is extremely tough and was used in centuries past to fashion tools such as knives, axes and clubs.

In jewellery terms, jadeite is the more valuable of the two varieties of jade — its value is simply a function of its beauty. It can be green, pink, lilac, white, brown and even red, but the most coveted shade is Imperial Green — a rich emerald colour.

Tiger’s eye, or chatoyant quartz, ranges in colour from brownish yellow to brown or reddish brown, depending on the presence of iron oxide. Of the many optical and physical properties that can affect the appearance of a gem and transform it into a ‘phenomenal’ stone, chatoyancy — the ‘cat’s eye effect’ produced by needle-like inclusions along a single axis point, which form a banded concentration of light — is particularly striking.

A variety of chalcedony quartz, onyx is banded with contrasting colours, which makes it an excellent medium for three-dimensional cameo and intaglio-style carving. It is also a term used for only black, non-banded microcrystalline quartz.

Black onyx was used by the Egyptians and the Romans, and became popular once more when incorporated into the mourning jewellery worn in mid-Victorian England.

In around 300 BC Theophrastus, a pupil of Aristotle’s, wrote a treatise on mineralogy in which he noted that the most beautiful samples of rock crystal came from the coldest mountain peaks. His theory that the mineral must be formed from water frozen at such extreme temperatures that it became impossible to melt again gave rock crystal its name — derived from the Greek word krystallos, meaning ‘ice’.

Craftsmen have revered rock crystal from as early as the Egyptian pre-dynastic era (5,000-3,000 BC), slowly cultivating their carving skills as they cut it into objects. It was not until the end of the 19th century, however, that jewellers introduced this clear variety of quartz to the exclusive world of luxury jewellery.

Discover colourful and unique hardstone, gold and diamond jewellery in Christie’s Jewels Online, 3-13 June