A massive (homogeneous and unlayered) siliceous rock, consisting of quartz crystals produced from quartz-rich sandstone when the sand grains and the silica that cements them together recrystallise. The crystals are milky white in colour, or orange because of the presence of iron oxides.
A grainy-textured plutonic rock formed by the crystallisation of magma deep within the earth and consisting essentially of quartz, potassium feldspars, plagioclase feldspars, and frequently micas.
Broadly speaking, any rock with a foliated (layered) structure, but the strict definition is metamorphic rock that has acquired a foliated structure through the effect of tectonic stresses (pressure). The micas are often aligned, the surfaces of the folia having a glossy or silky sheen.
Volcanic tufa, or tuff, is a rock with a porous structure. Calcareous tufa, also known as travertine, is a form of limestone and is of sedimentary origin.
A green, often dark green, metamorphic rock that is a constituent of ophiolites. It is also known as pot stone or soapstone and has long been used to make stoves and containers.
A sedimentary rock, like sandstone and gypsum, that is readily water-soluble (cf. karst), consisting mostly of calcium carbonate but also of magnesium carbonate. Where the rock has a significant proportion of clay (about 50%) it is known as marl.
A crystalline metamorphic rock characterised by dark-coloured layers (micas, amphiboles) alternating with light-coloured layers (quartz, feldspars). There are several different types.
A hard rock consisting of rounded or sub-rounded gravel and cobbles, cemented by limestone or silica.
Toutes les infos de l'étude des terroirs viticoles valaisans