Gold is so universally rare that normal stars are physically incapable of forging it. Yet Earth actually contains enough to coat the entire planet in a solid, foot-deep layer.
The cosmos is dominated by light elements like hydrogen and helium. While heavier elements are forged through nuclear fusion, standard stellar fusion hits a hard limit at iron because fusing anything heavier requires more energy than it releases. Creating heavy metals like gold requires an extreme mechanism known as the rapid neutron-capture process, or r-process.
In the r-process, an atomic nucleus is bombarded by free neutrons so quickly that it doesn't have time to radioactively decay before capturing the next one, rapidly building up into heavier elements.
The required density of free neutrons for the r-process is staggering, and the universe provides very few environments capable of supporting it. The primary forge for the universe's gold is the collision of two neutron stars—the ultra-dense, collapsed cores of dead giants. These mergers are incredibly violent but extremely rare, occurring only a few times per million years in a typical galaxy. Because the events that produce it are so scarce, gold remains a cosmic rarity.
If there is so much gold on Earth, why is it so scarce on the surface? When the early Earth was a molten ball, it underwent a process called planetary differentiation, often referred to as the iron catastrophe. Dense metals, including iron, platinum, and gold, sank toward the center of gravity, locking the vast majority of the planet's precious metals inside its core.
The tiny fraction of gold found in the crust today didn't originate with the planet's formation. It arrived millions of years later. Meteorites peppered the Earth's surface, delivering a "late veneer" of metals. The gold mined today consists entirely of the remnants of extraterrestrial impacts that dusted the planet long after its crust had solidified.