You have a beautiful gold coin, Hani, and it is valuable like all gold coins. Don't try to clean or polish it. That always ruins the value.
The coin in the picture comes from Ira & Larry Goldberg Coins & Collectibles. CoinQuest thanks the Goldbergs for the use of their picture. They have sold it for $350 USD US dollars, a good price for a nice, uncirculated 1894 coin in a PCGS numismatic slab. Most of the coins in this series command high prices when they are in superb numismatic condition. They fall lower, however, when worn, scratched, or heavily circulated.
A good way to estimate the value of these old gold pieces is to first compute their 'melt' value and then add the value due to numismatic (coin collector) appeal. These coins contain 0.1867 troy ounces of gold.
Using web sites such as kitco.com, find the current price of gold and multiply. At the time of this writing, the price of gold is around $1187 US dollars per ounce, so the melt value would be 0.1867 x 1187 = $221.61. Melt value is also called base value (BV).
Here are some typical total values for these coins, that is, the base value plus the numismatic value.
20 FRANCS ALL YEARS EXCEPT THOSE SHOWN BELOW:
average circulated: BV
well preserved: BV + $110
fully uncirculated: BV + $175
There are a few dates which cause the values to skyrocket. They are:
20 FRANCS BETTER DATES:
1887: $70000 in well preserved condition
1888: $8000 well preserved