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 coin type is the second from a series of ten coin designs known as the Queen's Beasts.
The Queen's Beasts are British coins issued by the Royal Mint in platinum, gold, and silver since 2016. Each of the coins in the series features a stylized version of one of the heraldic Queen's Beasts statues present at the coronation of Queen Elizabeth II representing her royal line of ancestry. It was announced that engraver Jody Clark will design the entire series; he is best known for also designing the Queen's Fifth definitive portrait, which is on the obverse of the coins.
The Queen's Beasts heraldic statues representing the genealogy of Queen Elizabeth II stood in front of the temporary western annexe to Westminster Abbey for the Queen's coronation in 1953. Each of The Queen's Beasts consists of an heraldic beast supporting a shield bearing a badge or arms of a family associated with the ancestry of Queen Elizabeth II. They were commissioned by the British Ministry of Works from sculptor James Woodford. The statues were uncoloured except for their shields at the coronation. They are now on display in the Canadian Museum of History. There are other statues of the Queen's Beasts, sometimes referred to as the King's Beasts, at Hampton Court Palace and Kew Gardens in London, and on the roof of St George's Chapel, Windsor Castle.
The Unicorn of Scotland comes from the Royal Arms of Scotland. From the end of the 16th century, two unicorns were adopted as the supporters of the Scottish Royal Arms. In 1603, the crown of England passed to King James VI of Scotland, who then became King James I of England. He took as supporters of his Royal Arms a crowned lion of England and one of his Scottish unicorns. The unicorn holds a shield showing the Royal Arms of Scotland, a lion ramping in a royal tressure, adorned with fleur-de-lis.
The silver one kilogram coin was issued in a mahogany presentation box, Royal Mint Product Code: UK17QUSK, issue price: £2050.00, with issue limit of 150 boxed, maximum coin mintage 225, in Proof FDC grade only. | 
Search for United Kingdom: Silver Kilo 2017 Unicorn of Scotland on eBay 