How Much Does It Take From The Value

Looks XF to AU (min) to me...even with the rim damage, you're looking at a 2000-4000 USD coin (if it is real).

My opinion, nothing more. I'd weigh it and magnet test it (it should be made of nickel) first, ensure that both of those aspects are correct.

If it's magnetic it's certainly a fake, hate to break it to you.

YES ITS SUPPOSD TO BE MAGNETIC IF REAL
I Wouldnt be sourprised if the chinese would make nickel replica's since its sutch a cheap metal
BUT I think in myt cas its obviouss that strike is geniuine

Both elements have a Mohs hardness of 4.

When counterfeiting a rare coin, counterfeiters sometimes use the proper metal (gold coins are known to be counterfeited by using an alloy of gold, silver ones with an alloy of silver...occasionally the proper alloy and real blanks by just over-striking the original coin), nickel is cheap compared to gold or silver, and with such a price for the real coins, I can see people using the real metals.

You could determine if the coin is of iron or nickel by doing a density test (I forget exactly how it is done, but google density testing of pure gold, and apply the same process using the densities of iron and nickel, wikipedia them). The coin, if real, is pure nickel...otherwise it would most likely be of pure iron, or it could be of some other alloy, but density will tell you for sure if it's nickel.

As to the value of it, if it were run of the mill, the value would be reduced dramatically. However, I could not find a mintage number at quick glance a few hours ago, but with the price of the coin I'd imagine the mintage is very low. And one thing I know for fact: low mintages drive up the value of the coin dramatically, and with it being the only in that series that is valued that high, slight rim damage and some scratches to the coin (actual degree of metal loss from wear is hard to determine by pictures) shouldn't detract much value, maybe slightly (i.e., a 4000 USD coin now becomes a 3800 USD coin)...it could be a lot worse, and that would detract tremendous value.

Thank you so muchfor your kind answers' I have not yet done the density test, I did the magnet, weight and measuring so far
A very common forgery is the aluminium -cooper edition witch is plated with a nikel or nickel alow, but that usualy is thiker and has 2.3 mm thikness!
All seems to be in order, I think is not hard to determinate a fake coin to a real one in this case

The coin should weigh 12 grams, have a thickness of 2.1 mm, have a diameter of 31 mm, and be made of nickel (which along with iron are the two most common magnetic elements). It's within an acceptable deviation I'd say for it to be real.

An aluminum-bronze alloy would weigh less than nickel (since nickel's atomic weight is 58.69, copper's is 63.54, and aluminum's is 26.98). Copper might weigh more, but aluminum weighs substantially less, so a mix of them would lower the weight significantly.

If the coin is iron, you should see the start of rust, depending on how it was stored in your grandfather's collection. If it was kept away from air (i.e., in a roll, in a bag buried under other coins, etc.) it would probably not show corrosion, but if it does then it is not nickel since nickel does not rust. It could be iron since iron's atomic weight is 55.84, and a slightly thicker coin would add the extra weight needed to bring the coin within range of a genuine one.

Density would tell you for sure if it's nickel or not.

You will need to examine the surface of the coin to determine if it is cast (google cast coins and find what to look for), but if it was die struck then you could compare it to a genuine 5 franc from another year (if you have one) and see if anything appears "off".

I'm hesitant to give you a yes/no answer with this, since it is a rare coin, I'm not entirely familiar with that series of francs, and since it's too easy to rush to a conclusion and believe it to be genuine. However, the more tests that you perform and that come out proper, it begins to look legitimate; however, it only takes one thing to be wrong and then it's definitively a counterfeit.

It might get a details grade, but you could submit it to a TPG: http://www.pcgseurope.com/Submit?l=en

A details grade, which although could lower the value theoretically by the fact that it's now truly marked as damaged, would at least determine it to be 100% genuine or 100% counterfeit. Of course, once it's graded, you could always crack it out of a slab if it comes back as genuine but damaged...that could raise the question of authenticity once more if you tried to then sell it, or you could keep it in the slab...it'd be up to you at that point.

I'm not sure about anyone else, but I'm thinking it could be real. Apart from sending it to a third party grader like PCGC, and getting an internationally recognized professional opinion, I don't think much more really could be done on you end (other than perhaps the density test or comparing it to another 5 franc from that series under magnification, to see that everything appears exactly the same, with the exception of the date, of course)

density test shows sometimes 8.95 sometimes 9.05-9.08 result
Another known nickel coin I haveeee shows 9.01 - 9.02