The outer covering of diagram is made of -
Reff: The synopsis of forensic medicine & Toxicology 28th edition pg: 195 Cell wall is usually strongly impregnated with silica and contain chlorophyll and diatoms a brown pigment. Diatom secretes hard siliceous outer box like skeleton called frustule. They resist heat and acid
Magnesium
Silica
Hydrocarbons
None
Asphyxia