Sandstone ... Sandstone rocks are sedimentary and made from small
grains of the minerals quartz and feldspar. As the layers built up, the
pressure increased turning the grains into stone.
It comes in many colours eg …honey, blonde, red, pink, white, grey. It has various grain sizes but medium or fine are probably best. It’s generally an easy stone to carve.
Slate ... Slate is a metamorphic rock, formed under great pressure from what was originally sedimentary mudstone or volcanic ash.
Slate comes in many colours …. black, blue, purple, red, green, or grey. Dark slates usually owe their colour to carbonaceous material or to finely divided iron sulphide. Reddish and purple varieties owe their colour to the presence of hematite (iron oxide), and green varieties owe theirs to the presence of much chlorite, a green micaceous clay mineral.
Caithness slab ... Caithness Stone is a unique material only found in the far north of Scotland. The stone was developed over 400 million years ago in a shallow tropical lake. Its fine particulate structure makes it comparable with mudstone or siltstone with a density and hardness akin to granite.
Its hard, fine grained nature means it will carve like slate but can splinter. Hence it takes more time to carve. It is naturally grey and in my view is best lightly oiled to bring out a more attractive dark, almost black colour. (see bonsai tree in the gallery)
Limestone ... Limestone rocks are sedimentary rocks made from the mineral calcite which came from the beds of evaporated seas and lakes and from sea animal shells. It is very fine grained but can have hollow pockets and nuggets/occlusions which can give some problems when carving. Traditionally a white-to-honey coloured stone but you can get black (grey when dry, black when wet). It is a soft stone, generally easy to carve and good for fine detail.