It is useless to try and discover an etymology for this word. It was invented wittingly. The most that can be said was that Van Helmont may have been thinking of the Dutch geest, a spirit; E. ghost.
This seems to us a strange selection; red, green, and violet would have been better. But this scale of colours is due to Aristotle, De Sensu, ii.; cf. Bartholomeus, De Proprietatibus Rerum, bk. xix. c. 7.
The Indian god Siva, was actually worshipped under the form of quicksilver. Professor Cowell refers me to Marco Polo, ed. Yule, ii. 300, and to his own edition of Colebrooke's Essays, i. 433; also to the semi-mythic life of Sankara Áchárya, the great reformer of the eighth century.
This explains why the alchemists, in seeking gold, sometimes supposed that they had obtained silver.
Archiv für das Studium der neueren Sprachen, ed. L. Herrig; vol. 86, p. 44.