It does not matter whichever material they are made from, except gold and silver, ornaments and jewelries are not subject to zakat. Therefore, any calculation is not required for them.
The calculation of zakat for 850 grams of gold is as follows: if you are not indebted, you calculate 2,5 percent of this amount and give it as zakat. According to this:
850 x 2,5 = 2125 / 100 = 21,25 grams
You can give this amount as gold or as cash.