Magnesite is a mineral with the chemical formula MgCO3 (magnesium carbonate). Iron, manganese, cobalt and nickel may occur in the stone but only in small amounts.
Stones of manesite are often dyed to look like turquoise and are often sold as turquoise, even though they are not even in the same mineral collection. Magnesite has the same hardness of turquoise and when magnasite is dyed with black lines running through it, is often difficult for the lay person to tell the difference.
The old adage, if it is to cheap it can't be real, should definitely be adhered to when it comes to turquoise and magnasite.
With so many unscrupulous jewelers and dealers, remember "buyer beware.