Crystal System: Hexagonal
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Aluminium oxide
Chemical Formula: Al2O3
Method(s) of Verification: All occurrences cited - verified by polarizing microscope.

Chemical Group:

  • Oxides & Hydroxides

Geological Context:

  • Igneous
  • Sedimentary: allogenic (detrital)
  • Metamorphic
Introduction: the mineral corundum, which includes the varieties ruby and sapphire, can be formed by both igneous and metamorphic processes. In igneous rocks it is typically found in silica-poor compositions, such as pegmatites or other rocks associated with nepheline syenites (Deer et al., 1992). It may also be found in aluminium-rich nodules in igneous rocks. Metamorphic corundum forms in silica-poor rocks in contact metamorphic zones and in thermally or contact metamorphosed bauxite deposits (Deer et al., 1992).
Occurrence in Wales: corundum has been recorded from altered rhyolitic lavas and rhyolitic tuffs from a borehole near Treffgarne, Pembrokeshire (Brown et al., 1987). Previously Heard (1922) identified prismatic-blue corundum crystals in sandstones and shales of the Pennant Series (Upper Coal Measures) of Carboniferous age exposed to the east of the River Taff, in South Wales.

Key Localities:

There are no key localities for this specimen.


  1. Brown, M.J. and others, 1987. Volcanogenic mineralisation in the Treffgarne area, south-west Dyfed, Wales. Mineral Reconnaissance Programme Report, British Geological Survey, No. 86.
  2. Deer, W.A., Howie, R.A. & Zussman, J., 1992. An Introduction to Rock-Forming Minerals. Longman Scientific & Technical, 696pp.
  3. Heard, A., 1922. The petrology of the Pennant Series, east of the River Taff. Geological Magazine, 59, 83-94.