Any British coin, or any coin from any of the 50 plus British Empire/Commonwealth countries might have had that inscription in 1949.
However, if there is no country name and the entire inscription as written appears on the "heads" side of the coin, it is most likely a British bronze Penny, Halfpenny or Farthing, or a nickel-brass dodecagonal Threepence.
There is not enough information given to answer. That's an abbreviated form of the Latin title of George VI (translated, it's "George VI, by the grace of God King of all Britain"), and would have been on most UK or British Commonwealth coinage in 1949.
"Georgivs Sextvs Rex" is Latin for "King George the Sixth". If the coin says only "Georgivs Sextvs Rex" on the obverse, it is likely not from Britain but rather one of its colonial holdings - the name of the colony would be on the coin somewhere. Any bank in this colony might then be interested in buying it, especially South Africa.
It is an Australian coin. George VI of England was also King of the Commonwealth countries.
This very much depends on condition, and what denomination the coin is. This can be checked on http://www.coins-of-the-uk.co.uk A coin of 1720 would say Georgius Rex or Georgivs Rex. George IV did not reign until the 19th century.
Most of the 25 cent coins are worth a price between $10 and $15 each. The price will vary depending upon the coin's condition.
Uh, did you look at the back of the coin where it says HALF dollar? See the Related Question for more details.
"Georgivs Sextvs Rex" is latin for "King George the Sixth". King George VI was King of Great Britain from 1936 to 1952. If the coin says only "Georgivs Sextvs Rex" on the obverse, it is likely not from Britain but rather one of its colonial holdings - the name of the colony would be on the coin somewhere. British coins from the reign of George VI usually have a somewhat longer phrase of abbreviated latin which translates to "George the Sixth, by the Grace of God, King of all Britons, Defender of the Faith [and until 1947] and Emperor of India." Most (although not all) coins bearing a portrait of George VI show him facing to the left and without a crown.
The inscription around the obverse of a current British Two Pence coin says - "ELIZABETH.II.D.G REG.F.D." and the year. "ELIZABETH.II.D.G REG.F.D." is abbreviated Latin for "Elizabeth II Dei Gratia Regina Fidei Defensor" meaning - Elizabeth II, by the Grace of God, Queen, Defender of the Faith. The reverse has the inscription "TWO PENCE".
I checked with P3M Refinings Payment Estimator. It says at the current silver price they will pay you $7.11 USD which is about $7.55 CAD at the moment. They keep 30% of what you coin is worth. So add those and you'll now what your coin is worth. It's about $10.78 CAD.
That's GEORGIVS, which is the Latin spelling of the English name George. The 1917 date would indicate that the coin is from the reign of King George V. However, you'll need to post a new question with the coin's denomination because ALL British coins have the King's or Queen's picture on them. Specifically, look for something with an unusual wording such as farthing, florin, half-crown, shilling, etc.
If the coin says Confederatio Helvetica, it is from Switzerland.
I do not think it will devalue the stone any, but it has certainly devalued the coin. Modified coins have no collector value.
It's a fairly safe bet that the coin is from Britain. The inscription is Latin and variations of it appear on all coins of the Empire / Commonwealth, with changes that refer to the reigning monarch of the time. In this case it refers to George III. If you can find the denomination or can make out enough details to post a description, please post a new question with that information. You don't need to repeat the motto; just note that the coin is British.