Lets say there are x diamonds, now these diamonds are exactly divisible by 7.
and
x = 1 + N1*2;
x = 1 + N2*3;
x = 1 + N3*4;
x = 1 + N4*5;
x = 1 + N5*6;
x = N6*7;
where N1, N2, N3, N4 and N5 are integers.
From above we can also say
N1*2 = N2*3 = N3*4= N4*5 = N5*6 = y
Now y should be divisible by 2, 3, 4, 5 and 6, its nothing but common multiple of all.
LCM of 2, 3, 2*2, 5, 2*3 => 2*3*2*5 => 60
at the same time common multiple + 1 should be divisible by 7 as well.
60 + 1 is not divisible by 7
120(60*2) + 1 is not divisible by 7
180(60*3) + 1 is not divisible by 7
240(60*4) + 1 is not divisible by 7
300(60*5) + 1 is divisible by 7
Thus they must have stolen minimum 301 diamonds.