Fluorescence is the visible emission of light which occurs when a diamond is exposed to ultraviolet light. It is a natural characteristic, observed in nearly one-third of all gem-grade diamonds, resulting from the presence of trace amounts of Nitrogen, a common impurity in diamonds.

The most common fluorescence colour observed in diamonds is blue, although other colours are possible, such as yellow, white, green and very rarely orange and red. In diamond certificates, fluorescence is usually classified as None, Faint, Medium, Strong or Very Strong.

Many people are falsely led to believe that fluorescence is a negative quality, when in fact it can have either a positive or a negative effect on the colour and overall look of diamonds. Diamonds with high colour gradings of D, E and F, exhibiting medium to strong fluorescence, can often look somewhat hazy or ‘oily’ in direct sunlight. However, those with relatively low colour gradings, around I or J, usually appear one or two grades better in colour due to the masking effect of blue fluorescence.

The effect of fluorescence on the value of a diamond depends entirely upon its strength and colour. The stronger the fluorescence the greater the negative effect can be on the value of higher colour grades and conversely, the greater the positive effect on the value of lower colour grades. The value of higher colour grades can be discounted by as much as 5% to 10% and the value of lower colour grades can be increased by a similar amount. Weak or faint fluorescence tends to have little or no effect on value.

As the name suggests, the emerald cut was originally popular for cutting emeralds. The emerald cut is typically rectangular, although it also comes as a square, called the square emerald cut. The Asscher cut, patented during the Art Deco period, is one version of this.

Reputed for its understated, sophisticated look, the emerald cut has less flash - reflection and refraction - than brilliant cuts; however, the larger facets are renowned for showing large, strong flashes of fire. The light return of these stones is less important than the effectiveness of the patterns and scintillation.

Because it is flat-sided, this cut lends itself to easy pairing with a range of side stone shapes. The quality of the diamond must be fairly high, as less faceting means inclusions are much easier to see - SI stones are often not eye clean. Colour also shows up rather too well, so it is advisable to look sideways, lengthways and diagonally when colour grading an emerald cut diamond.

A good length-to-width ratio is also important when selecting quality emerald cuts. While leading labs say a length should be between 1.5 and 1.75 times the width, lower ratios are also quite popular.

While blue is the most common colour of sapphires, it is found in everything from deep indigo to subtle violet, as well as pink, orange, green, brown and grey. Such derivations from the norm are called fancy sapphires; however a vivid, cornflower blue, verging on violet is considered the gem’s most valuable shade. Indeed a sapphire too dark or light in colour has a much lower value than one with more vivid tones.

Sapphire is a form of the mineral corundum - a colourless mineral formed from crystallised aluminium oxide. In fact, corundum in all colours belongs to the sapphire family except for red, which is called ruby.

Today, sapphires are mainly sourced from Thailand, Sri Lanka, Australia - the source of yellow and green varieties - Madagascar and Tanzania. Most of the fine blue sapphires on today’s market are from Sri Lanka and Madagascar. Kashmir, Burma,Thailand and Cambodia are also renowned for deep blue sapphires of limited supply.

Cut, colour intensity, consistency and purity are the deciding factors in a sapphire’s value. The finest sapphire should not contain any secondary colours or nuances because this will diminish its value and beauty.

Despite being a hard gem, rating nine on Moh’s scale, sapphire must still be treated with care. The gem is slightly brittle and can develop internal flaws and cracks if dropped on a hard surface or delivered a sharp blow.

Sapphire is the birthstone of September and the gemstone for the 5th and 45th wedding anniversaries. For its striking beauty and versatility, sapphire-set jewellery remains ever-popular.