It is demonstrated that a cone fracture can be produced in a brittle material by a liquid "indenter". Cone fractures of carbonatite structures can be interpreted on the basis of the Hertzian fracture theory. Tensile stresses are most important in initiating cones at shallow depths and shear stresses start cone fractures at great depths. Four stages of cone fracture can be observed in the Homa Mountain area. The spiral cone fracture at Homa Mountain and several other ring structures can be looked upon as an inherent feature of these structures and perhaps spiral cone fractures are more common than has been thought before. Twin centres can also be explained on the basis of the Hertzian theory.