The two senses of the word
The word marble carries two meanings at once. In the geological pages it is a limestone whose calcite has been recrystallised; the stone trade also uses the name for carbonate stones that take a good polish.
| In the geological pages | In the stone trade |
|---|---|
| A metamorphic rock. The Wombeyan plan’s glossary defines marble as limestone recrystallised and hardened by pressure and heat, and Heritage South Australia’s stone note lists marble among the metamorphic rocks. | A trade name too: Heritage South Australia’s stone note applies it to carbonate rocks that can take a good polish, and to fine-grained dense limestone and dolomite, which are sediments, as well as to true marble. |
A limestone, recrystallised
The Wombeyan plan puts the route plainly: “The intrusion of the igneous rock into the surrounding limestone metamorphosed the limestone into marble.”
The Australian Museum describes the chemistry the same way: in a pure limestone the calcite recrystallises and its grains grow coarser, so the rock becomes marble; an impure one can grow new minerals instead.
The stone note adds what minor impurities do at the surface a reader sees: they colour the stone, and they bring out the often diffuse pattern running through it.
What the stone is made of, at one Australian deposit
At Penrice, in South Australia, the state’s Department for Energy and Mining describes a marble deposit in the Early Cambrian Normanville Group: limestone reefs, later metamorphosed, recrystallised and folded.
The same South Australian department says the high-grade central part of the formation is 95 to 98 per cent calcium carbonate, flanked by darker lower-grade zones carrying contaminants and interbedded calc-silicates.
Water, and the slow dissolve
The karst reserve plan describes the same mineral meeting water underground: as the water passes through the soil it becomes slightly acidic, and in that state it dissolves the marble, slowly enlarging the cracks into passages and chambers.
Above ground the mineral is the same: the plan’s slightly acidic water and the guidelines’ refused acidic cleaners both meet calcite, the calcium carbonate mineral the Borenore plan’s glossary says limestone is mainly made of.
Marble and the hardness scale
The Australian Museum describes a relative hardness scale running from talc at the soft end to diamond at the hard end.
None of the official guides cited on this page places marble on that scale.
What the guides do say about scratching is practical: Heritage Victoria’s code warns that rubbing damages the substrate, because the treatment removes the outer layer of the material.
The same code places rubbing as a last resort, and the South Australian guidelines say wire brushes and metal tools are not used on a headstone.
Where this question goes next
Two guides sit beside this one: Why does acid damage marble?, and How do heritage guides say to clean marble?