Do Card Effects Stack?

I have a number of cards that grant a % more of a specific block type, if I have more than one of them chosen when I start digging will the effect stack or does it just choose the highest?

I actually got a reply from their twitter account when asking this question:

I don´t know if all card effects stack but the when I used Diamond salesman level 5 (Sell diamond nuggets for 150% more) and Diamond Industry level 3 (Sell diamond nuggets for 100% more. 25% chanse to drop an extra nugget) in the classic mode it said diamond (dont remember how many i got) and +250% så atleast the + value cards stack. You can try this yourself just start a game and pick one x salesman and x industry card (that boost the same mineral ofc) and then check the details, as I said earlier I don´t know if all card effects stack and I don´t know if they have changed that but it worked the last time i checked.