Shiva Purana
अभावे हेमरत्नानामशक्तौ च तदर्जने ॥ मनसैवाचरेदेतद्द्रव्यैर्वा प्रतिरूपकैः ॥ ६५ ॥
If liṅgas of gold and gems are not available or if he is unable to possess them he shall perform the worship mentally or with substitute articles.
english translation
abhAve hemaratnAnAmazaktau ca tadarjane ॥ manasaivAcaredetaddravyairvA pratirUpakaiH ॥ 65 ॥
hk transliteration by Sanscript