Assuming you mean a British George III Crown, it depends on the date and condition. Many of these coins are found with at least moderate amounts of wear and those in mint state or about uncirculated condition can fetch a premium, even for common dates.
1820 coins are worth the least, little more than bullion (about $25 or so at the time of writing) in worn condition or about $45 if in decent condition. Of course an uncirculated specimen can be worth several hundred dollars.
1818 and 1819 are worth moderately more, about $50 or so if in decent condition, although still little more than bullion if very worn.
However, the 1819 has a variety where there is no stops on the edge of the coin, this is worth about $55 or more depending on condition.
Being a short series, this is a pretty easy set to obtain a date run (only crowns dated 1818, 1819 and 1820 were produced for George III crowns, ignoring of course the Bank of England tokens and the "dollar" issues)
They made literally hundreds of different coins during George V's reign. Please post a new, separate question with the coin's date and country of origin.
About $35.
The value of a 1939 George VI British silver half crown can vary depending on its condition and wear, but ranges between $3 and $20.
There were no 1740 British Crown coins minted. The Crown coin was not regularly minted during the reign of King George II.
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.
Between $25 and $318. In the future please add a condition of the coin so I can value it more precisely for you.
Modified coins have no value as a collectible. Any value would be associated with the necklace.
There was no 1946 British Crown minted.
Between $16 and $60. In the future please add a condition of the coin so I can value it more precisely for you.
George Washington George Washington
The 1950 Half Crown of Georgius VI is not made of silver, but of cupro-nickel. 50% silver Hlaf Crowns were issued until 1946. Source: Wikipedia
There was no British Crown minted from 1938 to 1950 inclusive.
I have a vase with a crown on the bottom
Halfcrown coins do not have St George on them. Please have a closer look at your coin and post a new question. It is possible that you may have a crown and not a halfcrown. The crown's value would be anywhere from £15 (scrap value) to £350 in mint condition. Check the edge too to see which REGNAL YEAR (number of years the queen has been on the throne in Roman Numerals). It could be either LXIII or LXIV. The LXIII is rarer.