Jasper most commonly occurs when sediment uptakes silica from circulating water and builds microscopic quartz and chalcedony crystals in the in-between spaces of the substrate. Mud, limestone, diatomaceous earth, ash and any other stone which has porousness can become jasper. Jasper also forms when sediment mixes with silica rich circulating water which would normally form translucent chalcedony or agate but now contains up to 20% impurities making it an opaque stone.
Jasper ranges from a 6-7.5 on the Moh's scale of hardness and breaks with a chonchoidal fracture.
There are many kinds of legit jasper on the market and many stones marketed as jasper which are not actually jasper. Either because they are not quartz based or they are not opaque. Bumblebee "Jasper" is an an example of a material which is sold as jasper but is not jasper.
Jasper is a stone known since the ancient times and it is mentioned in many ancient texts including the bible. Jasper is known as a stone of grounding and stability, providing comfort and security, strength and healing.