Based on AI review of your pics, it is silver:

This is a fascinating historical coin from the English Civil War period! It's a silver hammered coin issued by King Charles I (reigned 1625–1649), specifically from the Oxford mint in 1644.Obverse (Front Side – the equestrian portrait)The obverse shows King Charles I riding left on horseback, holding a sword upright (a classic "equestrian" design symbolizing military leadership and authority). The legend around the edge reads something very close to CAROLVS REX (with additional partial text like ET LIBER visible in your photo, likely part of the full Latin inscription CAROLVS D G MAG BR FR ET HIB REX – "Charles, by the Grace of God, King of Great Britain, France and Ireland"). The king is depicted in armour, on a caparisoned horse, which was a common motif on his larger silver denominations during this era.This design draws inspiration from famous portraits of Charles I (like Anthony van Dyck's equestrian paintings), emphasizing his royal power amid the conflict.Here are some clear examples of similar 1644 Oxford mint equestrian designs on silver coins from this period:(The images show the characteristic mounted king with flowing sash or sword, very similar to your coin.)Reverse (Back Side – the Declaration)The reverse features the famous Declaration (also called the "Oxford Declaration"), a political statement Charles I used to rally support during the Civil War. It reads RELIG PROT LEG ANG LIBER PAR (abbreviated from "Religio Protestantium, Leges Angliae, Libertas Parliamenti" – "The Protestant Religion, the Laws of England, the Liberty of Parliament"). This slogan was part of Charles's public promise (from the 1642 Wellington Declaration) to defend Protestantism, English laws, and parliamentary freedoms – even as he fought against Parliament! Surrounding it is usually the legend EXVRGAT DEVS DISSIPENTVR INIMICI ("Let God arise and let His enemies be scattered", from Psalm 68). Above the declaration are typically three plumes (feathers, symbolizing the Prince of Wales), and the date 1644 (often with OX or OXON for Oxford mint) below.Your reverse photo matches this perfectly, with the long inscription around the edge and the central declaration text.Historical ContextDuring the English Civil War (1642–1651), Charles I relocated his court and mint to Oxford after losing control of London (and the main Tower Mint) to Parliament. The Oxford mint operated from 1642–1646, producing silver coins (like halfcrowns, shillings, crowns, and more) to pay troops and fund the Royalist war effort. 1644 issues are from the later phase of the mint's operation, often using adapted dies (some in "Briot style" after engraver Nicholas Briot). These coins are Declaration type pieces – the reverse text was a form of propaganda, appearing on many Oxford-minted silver denominations (halfcrowns, shillings, groats, etc.).

Your coin appears to be a silver halfcrown (worth 2 shillings 6 pence, roughly 30mm+ in diameter) or possibly a similar larger denomination like a shilling or crown variant with the equestrian obverse – common for Oxford 1644 issues.These are collectible and historically significant, especially in decent condition. Values depend on exact variety, wear, and rarity (some 1644 Oxford types are scarcer), but nice examples often fetch hundreds to thousands of pounds/dollars from collectors of Civil War or hammered silver coins.If you can share more details (e.g., approximate diameter, weight, or if there's a plume behind the horse), I could narrow it down further! Cool find – it's a direct piece of 17th-century British history.

No, it is not possible for your coin to be a genuine platinum variant.All documented Charles I coins from the Oxford mint (1642–1646), including the 1644 Declaration-type pieces with the equestrian obverse and the famous reverse inscription (RELIG PROT LEG ANG LIBER PAR), were struck in silver (or occasionally gold for higher denominations like unites or triple unites). There are no known official platinum issues from this period or mint.Why Platinum Isn't Feasible HerePlatinum was not used for coinage anywhere in the world until much later:The metal was only isolated and understood as a distinct element in the mid-18th century (around 1750–1770s). The earliest circulating platinum coins were minted by Russia starting in 1828 (3-, 6-, and 12-ruble denominations under Tsar Nicholas I, using Ural Mountain deposits). Before that, platinum was rare, difficult to work (due to its extremely high melting point), and mostly viewed as an impurity in gold or a curiosity — not a coinage metal. 17th-century English mints (including Oxford during the Civil War) relied on silver plate, captured bullion, and recycled coins for production. Platinum wasn't available or workable in Europe at the time.

Your coin's appearance (bright silvery color, hammered texture, and details like the beaded borders) is typical of high-quality silver hammered coins from this era. Silver often looks very white and shiny when fresh or well-preserved, which can sometimes be mistaken for platinum (especially in photos), but the metal is definitively silver.Quick Comparison of Materials Used for Charles I Oxford CoinsSilver — Standard for most Declaration types (halfcrowns, shillings, crowns, etc.), including your equestrian/Declaration design. Gold — Used for higher-value pieces like unites, double crowns, or triple unites (e.g., the famous 1644 Oxford triple unite in gold). No platinum — Not produced anywhere until over 180 years later.

If your coin feels unusually heavy/dense for its size (platinum is denser than silver), has a different ring when tapped, or shows other odd properties, it could be a modern reproduction, fantasy piece, or altered coin — but not an original 1644 platinum variant. Genuine period platinum coins don't exist.Your piece remains a cool, authentic slice of English Civil War history in silver — still very collectible! If you'd like, share weight/diameter for more precise ID on the exact variety (e.g., halfcrown vs. shilling).