Sea urchin injuries are puncture wounds inflicted by the animal's brittle, fragile spines. These materials are generally quite brittle and prone to fracture. Bronzes are typically ductile alloys, considerably less brittle than cast iron. Crocodilian eggs are somewhat brittle, but softer than bird eggs. During puberty, bones become harder and more brittle. Impact and tensile strength, scratch resistance, and resistance to brittle fracture are enhanced. Ice can be soft, hard, brittle or tough. Obsidian is hard, brittle, and amorphous; it therefore fractures with sharp edges. Whether polonium is ductile or brittle is unclear. Malnutrition is also known to cause hair to become lighter, thinner, and more brittle. These minerals tend to be soft, weak, brittle, and water-soluble. Usually, attempting to deform bulk diamond crystal by tension or bending results in brittle fracture. It is a steely-grey, lustrous, hard and brittle transition metal. They are hard, brittle, rigid, translucent and have low molecular mobility. Its flexibility prevents the brittle enamel fracturing.