Shiva Purana
हैमाभावे राजतं वा ताम्रजं शैलजन्तथा ॥ मृन्मयं वा यथालाभं जातुषं चान्यदेव वा ॥ ७७ ॥
If gold is not available silver, copper, rock or clay shall be used for making the phallic idol whichever is available. Even lac can be used.
english translation
haimAbhAve rAjataM vA tAmrajaM zailajantathA ॥ mRnmayaM vA yathAlAbhaM jAtuSaM cAnyadeva vA ॥ 77 ॥
hk transliteration by Sanscript