The unglossed headword phrase, according to Varro cited in Pliny's Natural History (36.14), was applied to Parian marble (pi 643, pi 644), on the basis of its supposedly being quarried by lamplight. It has no connection with the precious stone ruby (lambda 879).
[1] Lychnitos -- more authentically Lychnidos -- is the name of a city (and an associated lake) in ancient Illyria, present-day Ohrid in Albania/Macedonia; see Barrington Atlas map 49 grid C2.