Yes, most certainly desirable. Not rare, rare, rare (I have had a few over the years and certainly seen plenty) but it is a sought after piece. Interestingly, the UK seemed to get more than its fair share of these. Our theory has always been that they were shipped here and used as "Good Luck" tokens during WW1.
There are repros "out there" however, but you say "very old" so I guess you can trace it back for many years.
As to how valuable it is - well there are a number of variables.
The iridescence is very important. If it's vibrant and "electric" (like someone plugged it into the mains) then that's Good with a capital G. If it's kind-of silvery then it's not so desirable.
Condition is of course, very important - but you know that, of course.
Then there's the question of is it stippled or not. If it is stippled it has an extra "je ne sais quoi" in the desirability stakes. The iridescence sparkles that little bit more on stippled pieces.
And finally, the edge. If it's a ruffled edge, then it's more easily found. If it's a pie crust edge (crimped like granny used to do, with finger and thumb) then it's more exciting and more "money".
Has that helped? Putting an absolute figure on it is well nigh impossible. But it IS a good item to have.