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 celebrates the Sapphire Jubilee (65th anniversary) of accession to the throne of Queen Elizabeth II.
The Royal Mint says:
"In 1977 millions of people crowded the streets as Her Majesty The Queen made her way to St Paul’s Cathedral to begin her Silver Jubilee celebrations. Many millions more watched on television all across the Commonwealth, little knowing that we would go on to enjoy a Golden and later Diamond Jubilee. In 2017 The Queen will reach an unprecedented milestone of 65 years as Britain’s monarch - becoming the first and only British king or queen to celebrate a Sapphire Jubilee."
The reverse was designed by Gregory Kenneth Cameron, the Anglican bishop of the Diocese of St Asaph in Wales, who also designed The Last Round Pound.
The silver one kilo coin was issued in a presentation box, Royal Mint Product Code UK17QJSK, issue price £2,050.00, edition limit 250. | 
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