Uniform Strand
A strand where every pearl is the same size, within a tight tolerance.
Uniform strands are the modern default and are what most people picture as a pearl necklace.
Uniform is a tolerance, not an absolute. A 7mm strand is normally 6.5mm to 7.5mm, sorted so no step is visible.
The sorting is the cost. Matching a hundred pearls for size, colour, lustre and surface takes far more material than the finished strand contains.
When you are buying
Look along the strand at a shallow angle rather than straight on. Size steps and colour drift show up immediately in profile and are almost invisible face on.
Graduated
A strand where pearls increase in size from the clasp to the centre: small at the neck, largest at the throat.
Matching
How consistently the pearls within one piece agree on size, colour, lustre and shape. A grading factor in its own right.
Pearl Size
The diameter of a pearl in millimetres, measured across its widest point. Price rises steeply, not evenly, with size.