Noun
A mineral having a glassy fracture, occurring in translucent trapezohedral crystals. It is a silicate of alumina and potash. It is found in the volcanic rocks of Italy, especially at Vesuvius.
A leucoplast.
Source: Webster's dictionaryRocks that contain leucite or nepheline, either partly or a wholly replacing felspar, are not included in this table. Source: Internet
They may be of considerable size like the grey, rounded leucite crystals found on the sides of Vesuvius. Source: Internet