In Punchbowl Crater, where the rocks have been quarried, they have a brown to yellow colour. Cinders have red-black colours which can grade to yellow when they are hydrothermally altered, due to the formation of the glassy rock palagonite. Many of the erupted rocks have undergone various degrees of alteration, including the formation of zeolitic palagonite; minerals included in altered rocks include analcime, aragonite, calcite, chabazite, erionite, faujasite, gonnardite, gypsum, montmorillonite, natrolite, opal, phillipsite and thomsonite. In some vents, such as Diamond Head, the rocks are so heavily altered that their original composition and texture can no longer be reconstructed.