All That Glitters Is Gold
We'll start with the grand-daddy of them all: gold. Gold is unique for its durability (it doesn't rust or otherwise corrode), malleability and its ability to conduct both heat and electricity. It has some industrial applications in dentistry and electronics, but we know it principally as a base for jewellery and as a form of currency.
The value of gold is determined by the market 24 hours a day, nearly seven days a week. Gold trades predominantly as a function of sentiment; its price is less affected by the laws of supply and demand. This is because new mine supply is vastly outweighed by the sheer size of above-ground, hoarded gold. To put it simply, when the hoarders feel like selling, the price drops. When they want to buy, new supply is quickly absorbed and the gold prices are driven higher.
Several factors account for an increased desire to hoard the yellow metal:
Systemic Financial Concerns: When banks and money are perceived as unstable and/or political stability is questionable, gold has often been sought as a safe store of value.
Inflation: When real rates of return in the equity, bond or real estate markets are negative, people regularly flock to gold as an asset that will maintain its value.
War or Political Crises: War and political upheaval have always sent people into gold-hoarding mode. An entire lifetime's worth of savings can be made portable and stored until it needs to be traded for foodstuffs, shelter or safe passage to a less dangerous destination.