The tax authorities consider the ownership of precious metals as an investment. You must report your gold and silver in your annual tax return under box 3 (savings and investments), stating the value as of January 1st of the relevant year. You are responsible for declaring your precious metals.
 
There are exceptions for precious metals considered as collectibles, although this is rare. The European Commission has a list of coins that are always regarded as investment gold. These coins fall under box 3 for income tax purposes. For details on how your assets in box 3 are calculated, consult the tax authorities.
 

 
There is also an exemption scheme with two exemptions:

1. Exemption for items of art and science
2. Exemption for a collection (special form of art and science)