Agates are a common variety of brightly coloured quartz. They come in a range of colours depending on the mineral impurities they contain, and can be used in jewelry.
Alexandrite is a beryllium aluminium compound that is a different colour depending on which angle you look at it. The most prized colour combination is a range from green to purple.
Alum could refer to either a specific mineral, or a general class of similar ones. ( /?æl?m/) is both a specific chemical compound and a class of chemical compounds. The mineral usually referred to as alum is hydrated potassium aluminium sulfate. Alum is an effective blood coagulant, and used to be sold in blocks as part of shaving kits. You use it to stop shaving cuts from bleeding.
Amber is fossilised tree resin. It is a deep orange colour, and sometimes has insects trapped in it.
Amethyst is a violet variety of quartz, with the purple colour coming from trace amounts of iron and aluminium. Amethyst is a popular stone for making jewelry from.
Asbestos refers to a group of silicate rocks that have the interesting property of forming long, thin fibers. Asbestos was cheap, strong, had great insulating qualities and was widely used as a building material, with its use peaking in the 1970s. Asbestos can cause lung cancer, mesothelioma and other issues if it is inhaled, and because of this its use is currently banned in many countries.
Aventurine is a type of quartz which has impurities that make it green, translucent, and with a shimmering effect. Some types of aventurine are orange, brown, yellow, blue, or grey but the most common is green. Most of the world's green aventurine is found in India.
Basalt is a common rock made by lava cooling down quickly. Basalt is usually dark grey or black. Depending on how it cools down, basalt can be found in columns like in the picture, in a grid or hexagonal pattern, or even full of bubbles.
Beryl is a colorless mineral made up of beryllium aluminium cyclosilicate. It forms hexagonal crystals that can be very large. When beryl has impurities, it can have a coloured tint or be opaque. Coloured beryl has different names - green beryl is called emerald, pale blue beryl is called aquamarine and pink beryl is called morganite.