I have a question about whether the restriction on wearing gold (for men) applies to platinum as well. Is zakah applicable on platinum? And what is the reason for forbidding men to wear gold?
The Shari'ah has not forbidden the believers wearing gold. The traditions ascribed to the Prophet (sws) forbidding the use of gold for men should be interpreted as an application of another Qur'anic directive. The Holy Qur'an clearly declared extravagance a vicious act because it is this feature in a man, which eventually leads him to pomp and show. The Holy Prophet (sws) applied this Qur'anic directive to wearing of gold which was, and still is, a status symbol and a means of showing off ones wealth. However, since women have a natural desire to beautify them and adorn them with ornaments, the Prophet (sws) granted them allowance. Keeping the basis of the directive in mind one can safely conclude that not only gold but every other item which acquires the status of social symbol in a society and is used as a means for showing off wealth should be dealt with accordingly.
I believe that according to Shari'ah one has to pay Zakah on his total wealth, which is not in personal or business use. Precious stones no doubt are a part of your wealth; therefore they should be included in the total Zakatable wealth.