The question of whether gold is harder than platinum is common for anyone considering a significant jewelry purchase. The true answer is nuanced and depends entirely on the metal’s purity and how its physical properties are measured. Understanding the scientific definition of “hardness” and the effect of mixing metals is necessary for an informed decision. The metal that is genuinely harder in its pure form is often not the same metal that offers the greatest resistance to damage in a finished piece of jewelry.
Defining and Measuring Material Hardness
In materials science, hardness is defined as a material’s resistance to localized plastic deformation, such as scratching or indentation. This property is distinct from tensile strength, which refers to a material’s resistance to breaking under tension. Scientists use standardized metrics to quantify this resistance.
The Mohs scale of mineral hardness is a qualitative metric based on scratch resistance, ranging from 1 (talc) to 10 (diamond). While useful for general geological identification, it is not precise enough for metals used in manufacturing. For greater accuracy with precious metals, the Vickers hardness test is applied. This method involves pressing a diamond-tipped indenter into the material’s surface with a specific force, and the resulting indentation size is used to calculate a precise Vickers Hardness (HV) value.
Gold vs. Platinum: The Direct Hardness Comparison
When comparing the pure elemental forms of these metals, the difference between gold and platinum is clear. Pure gold (24-karat) is one of the softest metals, which is why it is rarely used in fine jewelry. Its Vickers Hardness value ranges approximately from 188 to 216 HV.
Pure platinum is significantly harder than pure gold, with a Vickers Hardness value generally falling between 400 and 549 HV. This difference means that, on an elemental level, platinum is more resistant to indentation and deformation than gold. Both metals are classified as noble metals due to their resistance to corrosion, but platinum’s atomic structure makes it inherently stiffer than gold.
Why Purity Matters: The Impact of Alloying and Wear
While pure platinum is harder than pure gold, jewelry is almost never made from the pure elements, making the comparison less practical for consumers. Alloying, the process of mixing a metal with one or more other elements, is necessary to increase the metal’s strength for everyday wear. The purity level of gold is expressed in karats (24K is pure), while platinum purity is usually expressed in parts per thousand, such as Pt950.
The addition of metals like copper, silver, or nickel dramatically increases gold’s hardness. For example, 14-karat gold (58.3% gold) is significantly harder than 18-karat gold (75% gold). The higher alloy content in 14K gold gives it a Vickers hardness ranging from 150 to 200 HV, compared to 18K gold’s range of 125 to 180 HV. This makes 14K gold the more practical choice for daily wear.
A similar effect occurs with platinum, where less pure alloys are often harder. Platinum 900 (Pt900) is slightly harder than Platinum 950 (Pt950) because the higher percentage of alloying metals, such as iridium or cobalt, increases resistance to indentation. Consequently, a gold alloy like 14K gold can sometimes be harder than a common platinum alloy like Pt950, which explains the confusion about which metal is tougher.
This difference in composition also highlights the distinction between hardness and overall durability. When platinum is scratched, the metal is not lost but is merely displaced, creating a subtle change in the surface called a patina. Gold, by contrast, loses a tiny amount of mass when scratched, which is a process known as material loss. Platinum’s characteristic of moving the metal around makes it more durable over a lifetime, as the piece retains its original weight and volume, even as it develops a softer, matte finish.