Specific gravity or "relative density" is a number that expresses the ratio between the weight of a substance and the weight of an equal volume of water at 4 degrees C. If a mineral has a SG of 2, then it is twice as dense as water. If a mineral has a SG of 3 then it is three times as dense as water and so forth. A substance with a SG of less than 1 will float. Calculating specific gravity requires special weighing apparatus that you may not have available in your lab. The goal here is in understanding that some minerals will "feel" heavier than others when in hand. The more dense a mineral is, the heavier it is per given volume. A 1 inch cube of galena is noticeably heavier in the hand than a 1 inch cube of pyrite. A Barite crystal of the same size as other similar non-metallic crystals is likely to feel noticeably heavier. So a mineral’s "heft" can be a clue to its identity. Generally, metallic minerals will have a higher relative density than non-metallics.
	Galena is a very dense metallic mineral.			Barite is a very dense non-metallic.
	Graphite is a very low density metallic mineral,			Opal is a very low density non-metallic mineral.