There are several types of Rock Wool designed for the propagation of seeds and cuttings. The best for hydroponic hobbyists is considered to be the Single Block System known as SBS.
This system consists of small individual cubes or mini blocks of Rock Wool on a small plastic tray. This makes the plants easy to handle and are ideal for selling or giving away.
Be certain to pre-treat the rock wool for 24 hours before use. Use a product that will reduce the alkaline nature of the media until it balances out at between 5.8 and 6.3.
One advantage of Rock Wool is its ability to hold enough water to last through the average period of time required for the cuttings to root. However, you will need to use a humidity dome.