Lapis Lazuli
An opaque deep blue rock flecked with gold pyrite. One of the oldest ornamental stones in use.
Lapis is a rock rather than a single mineral, coloured by lazurite and speckled with pyrite and white calcite.
The finest material, from Afghanistan, is an even royal blue with only a fine scatter of pyrite and little visible calcite.
It is soft and porous at 5 to 6 on the Mohs scale, so it is often waxed or dyed to deepen the colour.
When you are buying
Treat lapis like turquoise and pearl: keep it away from perfume, oils and ultrasonic cleaners, which strip the wax and lighten the colour.
Turquoise
An opaque blue to green stone, soft and porous. Called firoza in India and worn for Jupiter in astrology.
Semi-Precious
A commercial label for every gemstone outside diamond, ruby, sapphire and emerald. Not a measure of quality.
Onyx
A black chalcedony, usually dyed to an even colour. Common as a contrast bead beside white pearls.
Pearl Care
Pearls are soft and porous. They need to go on last, come off first, and be stored apart from everything else.