Jade
Two different minerals sold under one name: jadeite, the rarer and more valuable, and nephrite.
Jadeite and nephrite look similar and were treated as one stone for centuries. Imperial green jadeite is the most valuable form by a wide margin.
Both are exceptionally tough, meaning resistant to breaking rather than to scratching, which is why jade is carved rather than faceted.
The trade grades treatment by type: A is untreated, B is bleached and polymer impregnated, C is dyed.
When you are buying
Ask for the type letter. Type B and C jade look convincing when new and degrade over years, and the price gap against type A is enormous.
Gemstone
Any mineral or organic material cut for jewellery. Traditionally split into precious (diamond, ruby, sapphire, emerald) and semi-precious.
Semi-Precious
A commercial label for every gemstone outside diamond, ruby, sapphire and emerald. Not a measure of quality.
Cabochon
A smooth domed top with a flat base, no facets. The only way to show a cat's eye, star or opal fire.
Certification Labs
Independent gemmological laboratories that identify and grade stones and pearls. The name on the report is what gives it weight.