Rasendracudamani 11.4

11.4

{sulfur:: red}
raktaśca śukatuṇḍākhyo dhātuvādavidhau varaḥ /
{sulfur:: black}
durlabhaḥ kṛṣṇavarṇaśca sa jarāmṛtyunāśanaḥ //

And the red is called parrot's beak, excellent in the practice of alchemy;
And the black-coloured is rare, and it destroys old age and death.