Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Siphnus (-os), i, f., one of the Cyclades, now Sifano, Mel. 2, 7; Plin. 4, 12, 22, § 66; 36, 22, 44, § 159.
Hence, Siphnĭus, a, um, adj., Siphnian: lapis, Plin. 36, 22, 44, § 159.