How does slope on this graph indicate the amount of density?

The steeper the slope, the higher the density

Explanation:

A graph of mass against volume of different substances helps us to know that the slope is density since density is given by mass per unit volume. Therefore, the steeper the slope, the higher the density and vice versa.

luster is not a good characteristic to use by itself because luster is the property of minerals that indicates how much the surface of a mineral reflects light. the luster of a mineral is affected by the brilliance of the light used to observe the mineral surface.