Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† 1. pē̆tra, ae, f., = πέτρα, a rock, a crag, stone (pure Lat. saxum; cf.: rupes, scopulus): petrarum genera sunt duo, quorum alterum naturale saxum prominens in mare; alterum manufactum ut docet Aelius Gallus: Petra est, qui locus dextrā ac sinistrā fornicem expletur usque ad libramentum summi fornicis, Fest. p. 206 Müll. (of the latter signif. there is no other example known): petris ingentibus tecta, Enn. ap. Fest. 1. 1. (Ann. v. 366); Sen. Herc. Oet. 804: aquam de petrā produxit, Vulg. Isa. 48, 21 et saep.: gaviae in petris nidificant, Plin. 10, 32, 48, § 91: alga, quae juxta terram in petris nascitur, id. 32, 6, 22, § 66; 34, 12, 29, § 117; Curt. 7, 11, 1.