Natyashastra
शुद्धादर्शतलाकारं रङ्गशीर्षं प्रशस्यते । रत्नानि चात्र देयानि पूर्वे वज्रं विचक्षणैः ॥ ७३॥
The stage top should be of a clean, mirror-like shape, and precious gems should be placed on it, including Vajra (diamond) according to the experts.
english translation
zuddhAdarzatalAkAraM raGgazIrSaM prazasyate । ratnAni cAtra deyAni pUrve vajraM vicakSaNaiH ॥ 73॥
hk transliteration by Sanscript