The star rock cactus (Ariocarpus retusus) is one of the most highly sought after cacti, slow growing and hard to find. Comes in an 8cm pot.
 
Ariocarpus retusus is a species of cactus, from the genus Ariocarpus, found mainly in Mexico. It is one of the largest and fastest-growing species in this genus known for a slow rate of growth. Despite its slow growth, often taking ten years to reach flowering age, the retusus is a desirable cactus for cultivation, having attractive flowers and an unusual form for a cactus.