What is Gold?
Briefly here, gold is ambiguous term. It can refer to AU / the 79th atomic element gold, or it can refer to an alloy made from this element which also contain other metals.
24k gold is the element gold. Any lower karat number is a gold alloy where the element gold is mixed with other metals.
Most gold jewelry is made from gold alloy - 10k, 14k, 18k are all common.
How Is Gold Made?
If you thought JRR Tolken's ring death story was intense - consider the origin story of the element gold. It's forged not just in the heart of stars - but in the heart of supernovas, just before they explode under their gigantic mass.
Supernovas are formed by enormous stars when the process of nuclear fusion goes amok. Stars shine through fusion - the combination of two light elements into a heavier element.
A star starts as ball of hydrogen. Hydrogen fuses into helium until there is no hydrogen left. For sufficiently large stars this helium will fuse into carbon, then oxygen, neon, magnesium, silicon and finally iron. Here - again for sufficiently large stars - the star blows off most of its outer layers. However in the runaway fusion just prior to this explosion it creates gold (and other assorted heavy elements dependent upon the stars size) which is dispersed to nearby solar systems in a gigantic blast.
Theres a second known source of gold - sometimes the remnants of two supernovae (neutron stars) collide, and such collisions create even more gold. In August 2017 signatures of heavy elements including an estimated 3-10 earth size masses of gold were observed in the GW170817 neutron star merger.
Where Does Earths Gold Come From?
The center of the earth is estimated to contain enough gold to cover the entire surface a few feet high. The earth was born with this supply some four billion year ago. Most of this heavy metal migrated to the core when the earth was a liquid ball of rock - 99% of earths gold remains in the core to this day.
Lucky for us other rocks in our solar system were also seeded with gold, and some of these hit the earth after it solidified a bit giving us a bit more gold on the surface of the planet. For example a meteor impact about 3 billion years ago seeded the Witwatersrand mines - where some 40% of the gold we mine today originates.
How Long Will Gold Last?
Gold is overrepresented in museum collections for a reason.
From a human perspective: gold (element or alloy) lasts forever. Is almost non-reactive - it does not rush or decay or tarnish when exposed to oxygen (golds patina develops over decades and centuries). In its purest form gold is soft and prone to being dented or misshapen. In alloy form its stronger. 14k gold offers a good balance between durability and easy upkeep - 58% gold content imparts gold color and non-reactive properties while nickel, copper, silver, and zinc serve to strengthen to the metal.
All the gold on earth was created in supernovas, before the creation of our solar system, it became dispersed in the astroids and rocks from which the earth formed. The gold on earth will outlast all the stars you see in the sky (although there is some chance it will become incorporated into a red-dwarf sun some five billion years from now).
Why is Gold Precious?
Gold has been used for currencies and decor almost universally. Divergent cultures repeatedly decided that gold is precious. Why?
1) Gold was technologically simple. Of the seven metals known in antiquity, gold is the only one that occurred regularly in native form in the natural environment. One can mine for gold in river pebbles and find it in exposed nuggets.
2) Gold was hard enough to be useful but soft enough to be malleable.
3) Gold does not tarnish or degrade.