What mineral has 2 sugar-cube sized, irregularly shaped pieces. Brassy-yellow in color with metallic luster. Flat surfaces look like little squares. Feels quite heavy. Has a blackish streak.

The mineral we are talking about here is olivine.

Explanation:

It is a bit dark in color which ranges between yellow to Green to olive green. The luster present in it is nonmetallic luster but has a glassy finish and a substance hardness that is ranging between 6.5 to 7.

This mineral has granular masses which we can say has mass like sugar grains. This mineral has cleavage with conchoidal fracture present in it.