A karat mark is often read as a quality rating, with higher numbers assumed to be better. It is a statement of composition, and higher is not always more suitable.
The number is a fraction of pure metal
Karat expresses gold content in twenty fourths, so a twenty four karat piece is pure gold and an eighteen karat piece is eighteen parts gold to six parts other metals.
The remainder is not filler in any pejorative sense. It is a deliberately chosen alloy of copper, silver, zinc, nickel or palladium.
The same information is often shown as a fineness figure in parts per thousand, which is the convention used on bullion bars.
Pure gold is too soft for most jewellery
Gold in its pure state is remarkably soft and deforms under ordinary handling, which makes it unsuitable for settings that must hold stones securely.
Alloying hardens it substantially, and the hardness increases as the proportion of other metals rises.
A piece intended for daily wear therefore benefits from a lower karat, and jewellers select the alloy according to what the item has to withstand.
Alloy composition determines colour
Copper rich alloys produce rose and red tones, silver and palladium rich alloys move the colour towards pale yellow and white, and the proportions can be tuned finely.
Because colour depends on what is added rather than on how much gold is present, two pieces of identical karat can look entirely different.
This is why matching an existing piece requires knowing the alloy rather than only the karat mark.
Regional preferences follow different logics
Markets where jewellery serves partly as savings favour high karat pieces, because the value tracks the metal content closely and resale is straightforward.
Markets where jewellery is bought primarily as adornment favour lower karat alloys for durability, colour range and lower cost.
Neither preference reflects a judgement about quality; they reflect different purposes for the same object.
Resale is calculated from the mark
A buyer of scrap calculates value by multiplying weight by the fineness the mark declares, then deducting a refining and handling margin.
Because the calculation depends entirely on that mark, unmarked or doubtfully marked pieces are tested before any offer is made.
The craftsmanship, brand and original purchase price play no part in that arithmetic, which is why the metal value of a piece is usually well below what it cost.