Emerald

Emerald is the green variety of beryl, a beryllium aluminium silicate, coloured by traces of chromium and sometimes vanadium. The same mineral species gives aquamarine when coloured by iron, morganite when pink, heliodor when yellow and goshenite when colourless; they are one mineral wearing different impurities.

Emerald's defining practical characteristic is that it is almost always included. The internal fissures and wisps are so universal that the trade calls them the jardin, the garden, and treats an eye-clean emerald as suspect rather than desirable. The reason is geological: emerald requires beryllium and chromium, elements that occur in quite different rock types and rarely meet, so the crystals grow in disturbed, chemically hostile settings full of fractures. Beryl has a hardness of around 7.5 to 8, but emerald's fractures make it far more fragile than that figure suggests, and it is not a robust stone for daily wear.

Nearly all emerald on the market is oiled, fractures filled with cedarwood oil or resin to reduce their visibility. This is a long-standing and generally disclosed trade practice rather than a fraud, but it is a treatment, it degrades over time, and it is why emeralds should not be cleaned ultrasonically.

The historical sources were the Egyptian mines in the Eastern Desert near Mount Smaragdus, worked from the Ptolemaic period and traditionally associated with Cleopatra, producing generally poor material. The Spanish encounter with Colombian emeralds in the sixteenth century transformed the gem entirely; Muzo and Chivor still produce the finest stones, and Colombian emeralds flooded into Mughal India and Ottoman Turkey through Spanish trade, which is why great historical emeralds in Asian collections are usually American in origin.

The emerald's most systematic astrological role is not Western. In jyotisha, the Indian astrological tradition, emerald is panna, the gem of Budha, Mercury, and it is one of the navaratna, the nine gems, each assigned to one of the nine grahas.

The navaratna scheme is worth setting out because much of the gem material in circulation derives from it without saying so. The nine are: ruby for the Sun, pearl for the Moon, red coral for Mars, emerald for Mercury, yellow sapphire for Jupiter, diamond for Venus, blue sapphire for Saturn, hessonite garnet for Rahu, and chrysoberyl cat's eye for Ketu. Rahu and Ketu are the lunar nodes, not bodies, which is why the scheme has nine members rather than seven and why it cannot be mapped onto a Western planetary list without remainder.

This is a coherent, internally consistent system with a substantial textual and commentarial literature behind it, and it is still in active practical use: gems are prescribed by astrologers on the basis of a birth chart, with specific requirements about weight, metal, finger and the day and hour of first wearing. Whether or not one credits it, it is a living tradition with rules, not a modern assemblage.

Two cautions. The navaratna assignments do not agree with Western planetary gem lists, and where a modern source gives a stone a planetary ruler without saying which tradition it is drawing on, it is often silently mixing the two. And the antiquity of the full nine-gem scheme in its present form is less certain than usually implied; the gems appear in classical Sanskrit sources, but a fixed canonical navaratna set is harder to date than the confident presentations suggest.

Emerald is the May birthstone, and this is a good place to state plainly what the modern birthstone list is.

It is a trade standard from 1912. The National Association of Jewelers, the American body now called Jewelers of America, met at its annual convention in Kansas City in August 1912 and adopted an official list of birthstones. That list is the direct ancestor of the one printed on jewellers' charts today. It has been revised since, by committee, for commercial reasons: the Jewelry Industry Council of America added alexandrite for June, citrine for November and pink tourmaline for October in 1952 and replaced December's lapis lazuli with zircon; the American Gem Trade Association added tanzanite to December in 2002; and spinel was added to August in 2016. Britain's National Association of Goldsmiths issued its own separate list in 1937.

A scheme that acquires a new stone whenever a supplier has one to sell is a marketing instrument. That is not a hostile characterisation; it is what the revision history shows, and one critic quoted in the standard reference literature describes the 1912 list as nothing but a piece of unfounded salesmanship.

What makes this worth insisting on is that the 1912 list is almost always presented as ancient. It is not. There are genuinely old stone-and-month and stone-and-sign schemes, and they are different lists, they descend from attempts to correlate the twelve stones of the priestly breastplate with the twelve months and the twelve zodiacal signs, an idea found in Josephus in the first century and in Jerome in the fourth. Those are real antecedents and they deserve to be cited as such. The stones on the modern chart are not the stones on theirs.