shilohap,

ppm stands for parts per million, it is a relative number.

So if you give let us say a concentration of 25 ppm of gold per tonne, it means that there is 25 units of gold per 1 million units. In one metric tonne there are 1 million grams. Since there are 25 units you will have 25 grams of gold per 1 million grams of rock which is equivalent to 25 grams of gold per tonne of rock.

If you want to find how many grams gold there is in a kg of rock, 25 ppm means 0.025 g gold/kg.

Per gram of rock 25 ppm will get you 0.000025 grams gold/gram of rock

We must note that a short tonne and long tonne are not exactly 1 million, so conversion here is a little more tedious.

Bottom line: ppm equals g/tonne if dealing with metric