[Greek lithos, stone.]
This ending appears in adjectives formed from nouns in -lith: megalithic, otolithic. However, trilithic derives from trilithon (sometimes trilith), a structure consisting of two upright stones and a third across the top as a lintel, as in Stonehenge. Lithic exists as a standalone adjective, in reference to stone or the nature of stone.
The ending also occurs in adjectives and nouns relating to periods of the Stone Age; examples include Palaeolithic (US Paleolithic), the early phase, when primitive stone implements were used; Mesolithic (Greek mesos, middle), its middle part; Neolithic (Greek neos, new), the later part, when ground or polished stone weapons and implements prevailed.