Cabochon
A smooth domed top with a flat base, no facets. The only way to show a cat's eye, star or opal fire.
Cabochon is the oldest way of finishing a stone, and it is still the right one for anything valued for an optical effect rather than sparkle.
Chatoyancy in cat's eye, asterism in star sapphire and play of colour in opal all need a dome to gather and return the light.
It is also used for opaque and heavily included material, where faceting would achieve nothing.
When you are buying
For turquoise, opal, moonstone and anything with an eye or a star, cabochon is not a cheaper option. It is the correct one.
Cat's Eye
A stone showing a single bright band of light that moves across it, like a cat's pupil. The stone of Ketu.
Opal
A hydrated silica showing shifting spectral colour, called play of colour. Soft, water-bearing and fragile.
Turquoise
An opaque blue to green stone, soft and porous. Called firoza in India and worn for Jupiter in astrology.
Jade
Two different minerals sold under one name: jadeite, the rarer and more valuable, and nephrite.
Cut
How a stone is shaped and faceted. The one thing that most affects how it looks, and the one buyers examine least.