The headword is an adjective in the neuter plural. Very similar entry in other lexica (see the references in Photius kappa589 Theodoridis), but with particular differences noted below.
[1] (The Suda is wrong to repeat 'tough' in this definition; the other lexica have "growing together both amongst wheat and amongst pulses".) Besides Plato (below), kerasbo/la are also mentioned by Gregory of Nyssa, On the Soul and the Resurrection (PG 46.157 -- cited from the Nicene and Post-Nicene Fathers translation: see web address 1), and by Gregory of Nazianzus (Carmina Moralia 626). The latter writes: "there is a dry unsoakable seed, which never sinks into the ground, or fattens with the rain; it is harder than horn; its horn has struck the horn of the ox, what time the ploughman's hand is scattering the grain over his land."
[2] Odd accusative for "sowing and the plough" (presumably meaning "during sowing with the plough"). W.C. Greene's edition of the scholia vetera to Plato, where the same phrase occurs, considers this passage corrupt; so does Theodoridis loc.cit., who quotes Kuster's conjecture kata\ th\n spora\n kai\ to\n a)/roton.
[3] Plato, Laws 853D (web address 2). The word's metaphorical use in Plato is also commented on in Plutarch, Moralia [Quaestiones conviviales] 700C.