phenocryst
Concise Oxford English Dictionary © 2008 Oxford University Press:
phenocryst /ˈfiːnə(ʊ)krɪst, ˈfɛn-/
▶noun Geology a large or conspicuous crystal in a porphyritic rock.
– origin C19: from Fr. phénocryste, from Gk phainein ‘to show’ + krustallos ‘crystal’.
'phenocryst' also found in these Oxford entries:

