I hope it is not magnesium oxide!! That is useless and not absorbed.

Magnesium oxide remains the most common one out there because it is cheap. Doctors still prescribe it, and studies clearly show it is extremely poorly absorbed. From 500mg you might get 10mg into your blood stream.

Citrate and the delayed acting chlorides are better. I also use a powder sometimes called Ionic Fizz...it is very quick acting and quite nice.
It dissolves rapidly in warm water. I am quite sensitive to the laxative actions of magnesium, so I do the delayed type SlowMag.
I get the generic for it, at Costco, for about $5 for 60. (it is behind the counter at the pharmacy, and they will order it for you-- Mag64.
I just saw brand name SlowMag at WalMart last weekend on the shelf for 9.99 for 60. SlowMag was designed for use by physicians for cardiac patients and others, but it has always been over the counter.

I have a Vit D thread here also, and on it is new recommendations from a cardiologist who put on his blog, that people taking high dose D3 should cut back on calcium supplements (he does not recommend more than 600mg a day and only that for people with osteoporosis, etc)
He believes the Vit D greatly improves calcium absorption, and hence too much calcium may lead to hypercalcemia. (which then leads to arterial plaque and calcium deposits in the kidneys and other tissues).

If your D3 is only mixed in that calcium citrate, that is not considered high dose. High dose D3 is 2000IU and up a day.