geode
Concise Oxford English Dictionary © 2008 Oxford University Press:
geode /ˈdʒiːəʊd/
▶noun a small cavity in rock lined with crystals or other mineral matter.
■ a rock containing such a cavity.
– derivatives
geodic /dʒiːˈɒdɪk/ adjective.
geodic /dʒiːˈɒdɪk/ adjective.
– origin C17: via L. from Gk geōdēs ‘earthy’.
'geode' also found in these Oxford entries:

