Brucite

When cement or concrete are exposed to Mg²⁺, the neoformation of brucite, an expansive material, may induce mechanical stress in the hardened cement paste or may clog the porous system creating a buffering effect and delaying deterioration of the CSH phase into MSH phase. The exact magnitude of impact that brucite has over cement paste is still debatable. Prolonged contact between sea water or brines and concrete may induce durability issue although high concentrations are required for this effect, which are rare to find in nature. The use of dolomite as aggregate in concrete can also cause magnesium attack and should be avoided.

Beware of asbestos. Inhalation of asbestos fibres can lead to various dangerous lung conditions, including mesothelioma, asbestosis, and lung cancer. And it can take years for symptoms related to the exposure to appear. Lapidaries should exercise caution when cutting this mineral.