i think if government has not yet consider bitcoin as legal currency then you do not have to pay tax on gold or silver both investment is also illegla to use.
yes that will be right as we have to pay tax on legal things only and no country still consider bitcoin as legal currency therefore we do not have to pay tax on bitcoin therefore there will be no tax on silver or Gold if you are buying it using bitcoin.
if you are earning bitcoin then you don't need to pay tax because there are no clear regulation, however if you exchange it to fiat the government will consider it as income, you are going to be taxed, and in my country buying gold already being taxed, but if you are buying with bitcoin then it will no tax since the government can't track you