Shiva Purana
वैशाखे वज्रलिंगं तु ज्येष्ठे मारकतं शुभम् ॥ आषाढे मौक्तिकं विद्याच्छ्रावणे नीलनिर्मितम् ॥ ७३ ॥
In the month of Vaiśākha the Vajralinga is auspicious and in the eldest month the auspicious Marakata. In Ashadha it is known to be a pearl and in Shravana it is known to be made of blue.
english translation
vaizAkhe vajraliMgaM tu jyeSThe mArakataM zubham ॥ ASADhe mauktikaM vidyAcchrAvaNe nIlanirmitam ॥ 73 ॥
hk transliteration by Sanscript