Since the Recoinage Act of 1816 until 1919, all British Halfcrown coins were made from sterling silver with an Actual Silver Weight (ASW) of 0.4205 ounces.
The half-crown and crown are obsolete British coins. Under the old pre-decimal system, one crown was worth five shillings, and the half-crown was worth half that. At twenty shillings to the pound, the crown was 1/4 and the half was 1/8 of a pound. Or, the crown was 60 pence and the half was 30 pence.
The 100 ptas coin is a Spanish coin. The British Halfcrown coin has a face value of Two Shillings and Sixpence. To get a valuation on either coin, you would need to provide the date and condition of the coin.
Half sovereign - British coin - was created in 1544.
The value of the 1948 half crown ranges between 1 pound and 2.50 pounds.
Whatever British coins were being minted in 1916, were still being minted in 1918. The Half-Sovereign coin was not minted in 1917, but resumed minting in 1918. The Half-Sovereign was only periodically minted during the 20th century predecimal period, and was not minted at all from 1927 to 1936 and 1938 to 1979 inclusive. The British Third-Farthing coin ceased to be minted in 1913, although it was only minted specifically for Malta.
A Sixpence was a small predecimal British coin. Half a Sixpence was a Threepence, an even smaller predecimal British coin.
The value of a George VI half crown coin will depend upon its condition and how much the seller wants for it. You can expect to pay anything in the region of $1-$30 for this coin at auction.
Your coin is either a British George II gold Guinea or a Half-Guinea. The Guinea coin is 25-26 mm in diameter, the Half-Guinea a little smaller. Either coin in any condition would be worth something, if only for the value of the gold.
The design on the reverse of the Half New Penny coin was a crown.
If by "new" you mean a decimal coin, the Half New Penny and Half Penny coin was in circulation from 1971 to 1983. The 25 Pence (Crown) coin was only issued in 1972, 1977, 1980 and 1981 and was intended to be a commemorative rather than a circulation coin. The Five Pound Crown replaced the niche of the 25 Pence Crown in 1990. The uni-metallic Two Pound coin was issued in 1986, 1989, 1994, 1995 and 1996 and was intended to be a commemorative rather than a circulation coin. The general circulation bimetallic Two Pound coin was first issued in 1997.
The last British Half-Farthing coin was issued in 1868.
No US coin, including the half dollar has a crown stamp on it when it comes from the mint. Instead, if you have a US half dollar with a crown stamp in it, it was done either by a private company or individual. These stamps generally add no value to your coin and it is just worth what a damaged example of whatever year of half dollar you have is worth.