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  3. K18

* Gold hallmark

The K18 hallmark: meaning, purity & value

Japanese mark for 18-karat gold (75% by mass). The dominant bridal-jewelry standard in Japan together with Pt950. International equivalent is the 750 fineness mark. Color variant suffix (YG/WG/PG) typically follows, e.g. K18YG.

Published May 30, 2026

Quick facts

Metal
Gold
Purity
75.0%
Fineness
750/1000
Karat
18K
Common regions
Japan
Density
15.45 g/cm³
Melting point
905 °C
Standard
ISO 9202

Stamps that mean the same thing

This purity may be struck into jewelry as any of: K18 / 18K / 750 / 18金 / Au750. The mark differs by country and era, but the metal content is identical.

What K18 tells you

Japanese mark for 18-karat gold (75% by mass). The dominant bridal-jewelry standard in Japan together with Pt950. International equivalent is the 750 fineness mark. Color variant suffix (YG/WG/PG) typically follows, e.g. K18YG.

How to value it

The melt value of a K18 piece is gold spot price × 0.750 × weight (g). A buyer typically deducts 5–15% for assay, refining, and margin, so the cash offer lands just under that figure. Stones and complex settings are usually excluded from the metal weight.

How to check it yourself

  1. Examine the stamp under a 10× loupe — genuine marks are crisp and evenly struck, not doubled or smeared.
  2. Confirm the mark reads K18 or an equivalent such as 18K.
  3. Weigh the piece and estimate its volume — the density should land near 15.45 g/cm³ for this alloy.
  4. Photograph it in the Jewelry Identifier app to read the metal, hallmark, and any gemstones from the image.
  5. For a binding result, have an assay office or gemological lab run an XRF purity test.

Sources

* Frequently asked

FAQ

Q. Is K18 the same as 18K?
A. Yes. K18, 18K, 750, 18金, Au750 all denote the same material — 75.0% gold. Different markets and eras stamp it differently, but the purity is identical.
Q. How much is K18 worth?
A. Its melt value is the gold spot price × 0.750 × the weight in grams. Buyers then deduct roughly 5–15% for refining and margin, so a quoted buy-back price sits a little below that theoretical figure.
Q. How do I confirm a K18 stamp is genuine?
A. Look at the mark under 10× magnification for crisp, even strikes, cross-check the weight-to-volume ratio against the expected density (15.45 g/cm³ for this alloy), scan it with the Jewelry Identifier app, and — when it matters — have an XRF test done by an assay office or gemological lab.

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