Shiva Purana
कृते रत्नमयं लिंगं त्रेतायां हेमसंभवम् । द्वापरे पारदं श्रेष्ठं पार्थिवं तु कलौ युगे ७ ।
The phallic emblem of Śiva made of precious gems was considered the best in the Kṛta age; of pure gold in the Dvāpara; of mercury in the Tretā and of earth in the Kali age.
english translation
kRte ratnamayaM liMgaM tretAyAM hemasaMbhavam । dvApare pAradaM zreSThaM pArthivaM tu kalau yuge 7 ।
hk transliteration by Sanscript