more info on the actual link but below relative 'values':
The most common magnesium supplements are magnesium oxide, magnesium carbonate, chelated magnesium (magnesium glycinate), magnesium orotate, magnesium citrate, magnesium maleate and magnesium gluconate. These supplements provide different amounts of elemental magnesium (the constituent that matters) and also vary significantly in their bioavailability (absorption). Magnesium oxide is the most dense magnesium compound and the one most often used in mineral supplements and multivitamins. It contains 300 mg of elemental magnesium per 500 mg tablet, but is extremely poorly absorbed. Only about 4% of its elemental magnesium is absorbed or about 12 mg out of a 500 mg tablet.
Magnesium carbonate contains 125 mg of elemental magnesium per 500 mg tablet, but is poorly absorbed.
Chelated magnesium (magnesium glycinate) is magnesium bound in a complex of glycine and lysine. It is easily absorbed and highly bioavailable. The magnesium (elemental) content per tablet or capsule is usually 100 mg.
Magnesium orotate contains only 31 mg of elemental magnesium per 500 mg tablet. However, it is well absorbed and has been found highly effective in daily intakes of 3000 mg (186 mg elemental).
Magnesium citrate contains 80 mg of elemental magnesium per 500 mg tablet. It is far better absorbed than is magnesium oxide. The water soluble form (Natural Calm) contains 205 mg of elemental magnesium per teaspoon, is totally soluble in hot water and is highly bioavailable.
Magnesium maleate contains 56 mg of elemental magnesium per 500 mg tablet.
Magnesium gluconate contains 27 mg of elemental magnesium per 500 mg tablet. It is easily absorbed and quick acting.
All forms of oral magnesium supplements are better absorbed when taken with a meal.