Like many other mints, the Royal Mint issues bullion coins in precious metals, having much higher intrinsic value than their face value. Most of these by tradition are measured in troy ounces of fractions thereof, but the mint also introduced metric coins measuring one kilogram of pure metal (they are actually fractionally heavier as a whole, to account for the small amount of other metal in the alloy).
The 1 kg silver coins are denominated as Five Hundred Pounds (£500), although their intrinsic (bullion) value is much higher than their face value. The coins vary in design depending on the occasion (they are "one-year" types usually issued in parallel with smaller denominations with the same reverse designs).
The coins are legal tender but are not intended for circulation. They are targeted at bullion investors or collectors who appreciate the special editions as pieces of art.
This commemorative coin with £500 face value celebrates the Platinum Wedding (or 70th anniversary of the marriage) of Queen Elizabeth II to Prince Philip, Duke of Edinburgh.
The reverse was designed by John Bergdahl, and the obverse by Etienne Millner. The Royal Mint says about the design:
"To have fallen in love completely and unreservedly makes all one’s personal troubles and even the world’s seem small and petty." Written by Philip in 1946, these words demonstrate the strength of his love for Princess Elizabeth, which inspired him when designing her engagement ring. Diamonds from his mother’s tiara were set in platinum and The Queen still wears the ring to this day.
Etienne Millner’s combined portrait graces the obverse, while John Bergdahl’s double shield design features the heraldic lineage of the two families that became entwined when the couple married.
The coin is a massive one kilogram of silver (full weight of 1,005.00 g, 99.9% pure silver for an absolute silver weight of 1kg), struck to Proof FDC grade. It was sold by the Royal Mint in a mahogany presentation box; Royal Mint Product Code: UK17PWSK, issue price £2,050.00, Maximum Coin Mintage 550. | 
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