Gold, silver and cash

Q: I would like to query regarding 18k gold plated necklace. 

I previously was not aware that zakaat was due on my 18k gold plated necklace. I'm not sure how long I had it in my possession. I am just going to estimate 13 years. I recently also gave the necklace away within this current year. 

I am not sure how much the necklace weight is: will estimate 18 grams, since I am not sure of the percentage/weight of the "gold" in it. 

Will it be okay if I calculate the zakaat on the current selling price of the necklace e.g. The current selling price is R1200 and then to make up for the 13 years =R 15 500 and then pay 2.5% of this amount, will this be allowed ?

Q: I have been married around 15 years, and my wife was give more gold ornaments by their parents! There is zakaat to be given for it and she did not provide it! I don't have any gold or savings! Only my wife's gold! How can I calculate the zakaat for the past 15 years => Gold price was low to high till now. Do I need to calculate the yearly gold rate on that year! Can tell me how to discharge the zakaat for the past years?

Q: If I possess gold and cash (and no silver) the value of which goes above silver nisaab but remains below the gold nisaab, do I have to pay zakaat on it?

Q: If in 2017 and 2018 a person followed the silver nisab but happened to not pay Zakah in those years despite meeting the silver Nisaab then at a later point (maybe 2018 onwards) he moved to the gold nisaab and when he did calculation for past Zakah, he used the gold nisaab retroactively, even for those years where he followed silver nisaab but that meant for those years he did not owe Zakah according to gold nisaab. Would he be required to still pay Zakah for those two years because at the time of those two years he was following silver nisaab?

Q:

1. I wanted to ask if the only excess wealth a person has is a Kruger coin, is one liable for zakaat as it is less than 87 grams?

2. If a person has the cash that is more than the nisaab and also has gold in the form of 1 Kruger coin only, is zakaat liable on both the cash and the Kruger coin or only the cash?

Q: My wife has around 17 tola gold. I want to know how much zakat she has to pay on it? 

Q: If a person has a gold tooth, will it be necessary for him to include it in his zakaat calculation and pay zakaat on it?

Q: If jewellery is of unknown metals (unknown carat for example), how do we pay zakaah on it if there is no indication of what it is? If it's gold in colour but unknown what it consists of.

Q: My wedding jewellery set is now lost for two years. Until last year, I paid zakaat on it thinking that I would find it. At present, it is still lost. Should I continue to pay zakaat on it?