Typically the reaction to form Grignard reagents involves the use of magnesium ribbon.  All magnesium is coated with a  passivating  layer of magnesium oxide, which inhibits reactions with the organic halide. Many methods have been developed to weaken this passivating layer, thereby exposing highly reactive magnesium to the organic halide.  Mechanical methods include crushing of the Mg pieces in situ, rapid stirring, and sonication.  Iodine, methyl iodide, and 1,2-dibromoethane are common activating agents.  The use of 1,2-dibromoethane is advantageous as its action can be monitored by the observation of bubbles of ethylene.  Furthermore, the side-products are innocuous:
The amount of Mg consumed by these activating agents is usually insignificant. A small amount of mercuric chloride will amalgamate the surface of the metal, enhancing its reactivity.  Addition of preformed Grignard reagent is often used as the initiator.
Specially activated magnesium, such as Rieke magnesium, circumvents this problem.The oxide layer can also be broken up using ultrasound, using a stirring rod to scratch the oxidized layer off, or by adding a few drops of iodine or 1,2-Diiodoethane. Another option is to use sublimed magnesium or magnesium anthracene.
"Rieke magnesium" is prepared by a reduction of an anhydrous magnesium chloride with an potassium: