Melocactus azureus v. ferreophilus Mesemb In high light they have

In high light they have a gray cast

neohumberii

Forms big clumps in time

It is leafless and produces long blue-green stems with dark patterned markings

this is a rarely seen variety of this popular species

Melocactus azureus v. ferreophilus Mesemb In high light they haveMelocactus azureus var. ferreophilus This is a nice cephallium forming species with green to blue green skin. It looks and grows like normal cactus several years but once it gets big enough the main plant stops growing but produces a large hairy inflorescence that continues to grow for the rest of the life of the plant. Eventually the inflorescence can be taller than the plant itself. This is more tropical than most succulents so it doesn't like cold