From A Wiki of Ice and Fire
The tears of Lys is a rare and expensive poison. It is clear, sweet as water and leaves no trace. It is dissolved into liquids. The effects of the poison cause a sickness in the bowels and belly, eating away at them. According to semi-canon sources it is created by the alchemists from Lys.
A Game of Thrones
It was used to poison Jon Arryn. His wife Lysa Tully put the tears of Lys in his wine at the behest of Petyr Baelish.
||“The tears of Lys, they call it. A rare and costly thing, clear and sweet as water, and it leaves no trace. I begged Lord Arryn to use a taster, in this very room I begged him, but he would not hear of it. only one who was less than a man would even think of such a thing, he told me.” 
, to Ned
References and Notes