Technical differences
Origin, carat weight, shape, colour, clarity and cut information affect how offers compare. Exact millimetre dimensions and the individual appearance can matter as much as a small difference in a headline grade.
Commercial differences
A price also belongs to a particular offer: its currency, availability, documentation, delivery and validity. Compare equivalent scopes and confirm any extra costs. Decide which differences improve the result you want rather than assuming every higher grade is worth paying for.
Sources and further reading
This English edition draws on the source material referenced below.