Spinel, Tourmaline, Morganite and Tanzanite

Spinel is one of the great underrated gems. For centuries, red spinels were mistaken for rubies: the Black Prince’s Ruby in the Imperial State Crown is in fact a spinel. Today spinel is valued in its own right, in red, vivid pink, lavender, grey and, rarely, cobalt blue.

At 8 on the Mohs scale and with good toughness, spinel wears very well in an engagement ring. It is usually untreated, and its colours are bright and lively. Fine spinel comes from Myanmar, Sri Lanka, Tanzania and Vietnam. It was added as a birthstone for August in 2016.

Tourmaline grows in more colours than almost any other gem: greens, pinks, reds, blues and stones that show two colours at once, such as watermelon tourmaline with a pink heart and a green rim. Many tourmalines show a different shade when viewed along the length of the crystal, so a skilled cutter orients the stone for its best colour.

The most celebrated is Paraíba tourmaline, a neon blue to green coloured by copper, first found in the late 1980s in the Brazilian state of Paraíba. Similar copper-bearing stones have since been found in Mozambique and Nigeria. Brazil remains a major source of tourmaline of every colour.

At 7 to 7.5 on the Mohs scale, tourmaline suits a ring with a protective setting and sensible care. It is a birthstone for October.

Morganite is the pink to peach variety of beryl, coloured by manganese. It was named in 1911 in honour of the American financier J. P. Morgan. Its soft, warm colour is especially beautiful in rose or yellow gold.

Many morganites are gently heated to remove a yellowish tint and reveal a purer pink. The stones are often eye-clean, and the colour shows best in larger sizes. At 7.5 to 8 on the Mohs scale, morganite is a sound choice for everyday wear. Brazil and Madagascar are important sources.

Tanzanite is a blue to violet variety of the mineral zoisite, discovered in 1967 in the Merelani Hills of northern Tanzania, still the only known commercial source. Almost all tanzanite is heated to develop its blue-violet colour, and the stone shows different shades of blue, violet and burgundy as it moves.

Tanzanite is the softest of these four, at 6 to 7 on the Mohs scale, and it has distinct cleavage. In an engagement ring it needs a protective setting and gentle wear; it is wonderful in earrings and pendants. It is a birthstone for December.

Choose by colour first, then match the stone to the life the ring will lead. Spinel and morganite wear well every day. Tourmaline and, especially, tanzanite reward a protective setting. Ask for a report from an independent laboratory confirming the stone’s identity and any treatment, and care for it gently.

Clean it with warm water, a drop of neutral soap and a soft brush, then rinse and dry with a soft cloth. Never put a gemstone in an ultrasonic cleaner or a steamer.