It will be either a British Sovereign or Half-Sovereign.
That motto is on ALL British coins so it doesn't help to ID a specific coin. Please post a new question with the coin's denomination and an estimate of how worn it is.
Well, you didn't specify the denomination, but let me tell you what the legends mean. 1896 is obviously the date on the coin, Victoria is who is depicted on the front of the coin. Dei Gratia means by the grace of god. Britt Regina means queen of the British. Fid def means defender of the faith, and Ind Imp means Empress of India. Put these together and you get part of Queen Victoria's royal title, Victoria, by the grace of God, queen of Britain, defender of the faith and empress of India. As for the mintmark M, this mintmark would only appear on the gold coins, the sovereign and the half sovereign for this date. The mintmark would be above the date on the reverse between the 8 and 9 on the ground with St. George slaying the dragon. This mintmark means the coin was minted in Melborne.
Please look at the coin again, the US did not coin $25 dollar denominations in 1896.
At the very least, it will be worth the current bullion value of whatever gold and silver was used to make it. Presumably there is a coin on the chain. Whatever collector value the coin may have had was lost when the coin was modified as a piece of jewellery.
The last year for a gold US one dollar coin was 1889. If you have an 1896 Morgan silver dollar that has toned a gold hue, it needs to be seen for a accurate assessment. Take it to a coin dealer.
All Sovereign coinage issued by the Royal Mint since 1817 is 22 carat gold. These include the Five Pound (Quintuple Sovereign), Two Pound (Double Sovereign), Sovereign, Half-Sovereign and, in 2009, the Quarter-Sovereign.
The coin could be a gold Five Pound, Two Pound, Sovereign, Half-Sovereign or a silver Crown. In the absence of a denomination, the year, metal and diameter of the coin would help in identifying the coin.
Such a coin does not exist. Queen Victoria was not born until 1819 and coins with her likeness were not issued until 1838.
Your coin is either a gold Sovereign or a Half-Sovereign. A Sovereign is about 22mm in diameter and a Half-Sovereign about 19.3mm. The mintmark will be a small M, P or S indicating that the coin was minted at the Melbourne, Perth or Sydney Mint. Coins minted at the London Mint have no mintmark.
It is quite possible. These coins are a target for forgers. The 1893 Two Pound (Double Sovereign) coin should show the "Old veiled bust" of Queen Victoria on the obverse and St. George slaying the dragon on the reverse. The inscription should read - "VICTORIA DEI GRA BRITT REGINA FID DEF IND IMP" The coin is made from 22 carat gold and should weigh 16 grams and be 28.4 mm in diameter. If in doubt, take your coin to a reputable coin dealer for examination.
15 years is a very big mistake especially for a gold coin. I suggest that the coin is not what it appears to be. A reputable coin dealer will be able to identify your coin and give a valuation if it turns out to be genuine.
$475 to $550 for its gold content. Worn coins would be at the low end of that range.