Magic - Igneous rocks composed mostly of dark, iron- and magnesium-rich minerals. Magma - The molten material deep in the Earth from which rocks are formed. Magmatic segregation - An ore-forming process whereby valuable minerals are concentrated by settling out of a cooling magma. Magnetic gradient survey- A geophysical survey using a pair of magnetometers a fixed distance apart, to measure the difference in the magnetic field with height above the ground. Magnetic separation - A process in which a magnetically susceptible mineral is separated from gangue minerals by applying a strong magnetic field; ores of iron are commonly treated in this way. Magnetic susceptibility - A measure of the degree to which a rock is attracted to a magnet. Magnetic survey - A geophysical survey that measures the intensity of the Earths magnetic field. Magnetite - Black, magnetic iron ore, an iron oxide. Magnetometer - An instrument used to measure the magnetic attraction of underlying rocks. Map-staking - A form of claim-staking practiced in some jurisdictions whereby claims are staked by drawing lines around the claim on claim maps at a government office. Marble - A metamorphic rock derived from the recrystallization of limestone under intense heat and pressure. Margin - Cash deposited with a broker as partial payment of the purchase price for any type of listed stock. The stock is held by the broker as security for the loan. Marginal deposit - An orebody of minimal profitability. Market order - An order to buy or sell at the best price available. In absence of any specified price or limit, an order is considered to be “at the market”. Matte - A product of a smelter, containing metal and some sulphur, which must be refined further to obtain pure metal. Metallurgical coal - Coal used to make steel. Metallurgy - The study of extracting metals from their ores. Metamorphic rocks - Rocks which have undergone a change in texture or composition as the result of heat and/or pressure. Metamorphism - The process by which the form or structure of rocks is changed by heat and pressure. Migmatite - Rock consisting of thin, alternating layers of granite and schist. Mill - A plant in which ore is treated and metals are recovered or prepared for smelting; also a revolving drum used for the grinding of ores in preparation for treatment. Milling ore - Ore that contains sufficient valuable mineral to be treated by milling process. Millivolts - A measure of the voltage of an electric current, specifically, one-thousandth of a volt. Minable reserves - Ore reserves that are known to be extractable using a given mining plan. Mineral - A naturally occurring homogeneous substance having definite physical properties and chemical composition and, if formed under favorable conditions, a definite crystal form. Muck - Ore or rock that has been broken by blasting. Muck sample - A representative piece of ore that is taken from a muck pile and then assayed to determine the grade of the pile.   Prospecting &  Mining Basics