Karat vs Carat — What's The Difference?

Carat — A Measure Of Gemstone Weight

Carat, when applied to a gemstone, is a unit of weight. One carat equals exactly 0.2 grams — a standardised measurement adopted internationally in 1907 to replace the inconsistent regional weight standards that had previously made gemstone trading unnecessarily complicated.

The word derives from the carob seed — Ceratonia siliqua — which was used as a counterweight on balance scales in ancient gem markets because of its remarkably consistent weight from seed to seed. The modern carat has nothing to do with carob seeds, but the etymology survives.

What Carat Weight Tells You — And What It Does Not

Carat weight tells you how much a stone weighs. It does not tell you how large it looks, how beautiful it is or what it is worth.

Two sapphires of identical carat weight can appear dramatically different in size depending on their cut proportions and shape. A well-cut oval sapphire distributes its mass efficiently across the face of the stone, appearing larger than a poorly cut round of the same weight whose mass is hidden in excessive depth. An elongated shape — oval, pear, marquise — will always appear larger face-up than a round of the same carat weight because the elongated silhouette covers more surface area.

The Size Premium For Larger Stones

One important thing carat weight does tell you is where a stone sits in the size premium structure of the market. Fine gemstones are rarer in larger sizes — the geological conditions that produce large, clean, well-coloured crystals are considerably less common than those that produce smaller ones. As a result, price per carat increases as stone size increases, and the increase is not linear.

A fine two-carat sapphire does not cost twice what a comparable one-carat stone costs. It costs significantly more — often three to four times as much per carat — because the two-carat stone is considerably rarer. This size premium accelerates further for stones above three carats, and at five carats and above, fine quality natural sapphires become genuinely scarce and priced accordingly.

Karat — A Measure Of Gold Purity

Karat, when applied to gold, is a measure of purity — specifically, the proportion of pure gold in a metal alloy, expressed as a fraction of twenty-four parts. Pure gold is 24 karat — twenty-four parts gold out of twenty-four, or one hundred percent pure. But pure gold is too soft for most practical jewellery applications. It scratches easily, bends under normal wearing pressure and cannot hold gemstone settings securely over time.

24 Karat — 99.9% Pure

The purest form of gold available in jewellery. Rich, deeply coloured and entirely unmixed. Too soft for most jewellery applications — it scratches, bends and deforms easily under normal wear. Used primarily for investment bullion and certain ceremonial pieces.

22 Karat — 91.7% Pure

Very high purity with a deep, rich colour. Common in South Asian jewellery traditions where high gold purity is culturally significant. Not typically used for gemstone-set jewellery in Western markets due to its softness.

18 Karat — 75% Pure

The standard of choice for fine jewellery in most international markets, including Australia. Eighteen parts gold out of twenty-four — seventy-five percent pure gold alloyed with twenty-five percent other metals. 18ct gold achieves the optimal balance between gold purity and practical durability for daily wear. At Ruhuna Gemstones, all of our pieces are made in 18ct yellow gold — a choice that reflects our commitment to materials that match the quality and longevity of the natural sapphires they hold.

14 Karat — 58.3% Pure

Common in North American markets. More durable than 18ct due to the higher proportion of alloying metals, but noticeably paler in colour and carrying less gold content by weight. The colour difference between 14ct and 18ct yellow gold is visible to most buyers — 14ct has a cooler, less saturated yellow.

9 Karat — 37.5% Pure

The minimum gold content permitted to be sold as gold in Australia and the United Kingdom. Considerably harder and more durable than higher karat alloys, but significantly paler in colour and carrying less than half the gold content of 18ct. Not appropriate for fine jewellery intended for heirloom use.

Gold Colours — What The Alloy Determines

The colour of gold jewellery is determined not by the karat but by the specific metals used in the alloy. The same 18ct purity can produce yellow gold, white gold or rose gold depending on what is mixed with the pure gold.

Yellow Gold

The natural colour of gold, preserved through alloying with silver and copper. Yellow gold is the traditional choice for fine jewellery and the metal most closely associated with the sapphire in historical and contemporary jewellery design. The warm tone of yellow gold complements the colour of most sapphires — particularly blue, teal and pink — in a way that white metals rarely match.

White Gold

Gold alloyed with white metals — typically palladium or nickel — and almost always finished with a rhodium plating that gives it its bright, mirror-white appearance. The rhodium plating is not permanent — it wears off over time and requires periodic re-plating to maintain the white appearance.

Rose Gold

Gold alloyed with a higher proportion of copper, which produces the characteristic warm pink tone. Rose gold pairs beautifully with pink and padparadscha sapphires in particular, creating a harmonious colour relationship between metal and stone.

Platinum

Platinum is not gold at any karat — it is an entirely different metal. Naturally white, extremely dense and genuinely rare, platinum is used in fine jewellery as an alternative to white gold, offering a naturally white colour that does not require rhodium plating. Platinum jewellery is typically marked 950 or PT950 — indicating ninety-five percent pure platinum.

Reading A Hallmark

Fine jewellery sold in Australia and most international markets carries a hallmark — a small stamped mark that identifies the metal purity of the piece.

Why Both Terms Matter For Your Purchase

When you are evaluating a sapphire engagement ring, both carat and karat contribute to your understanding of what you are buying and what it is worth.

The carat weight of the stone — combined with its colour, clarity, origin and treatment status — determines the stone's value and its place within the sapphire market. The karat of the gold — combined with the quality of the setting's construction — determines the quality and longevity of the metal component of the ring.