In the course of time Cain, brought some of the fruits of the soil as an offering to the Lord. But Abel brought fat portions from some of the firstborn of his flock. The Lord looked with favor on Abel and his offering, but on Cain and his offering he did not look with favor. So Cain was very angry, and his face was downcast.

Now Cain said to his brother Abel, "Let’s go out to the field." And while they were in the field, Cain attacked his brother Abel and killed him.

The Lord said, "What have you done? Listen! Your brother’s blood cries out to me from the ground. Now you are under a curse and driven from the ground, which opened its mouth to receive your brother’s blood from your hand.

Genesis 4:3 - 5, 8, 10 - 11

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The altar of sacrifice is represented and since Cain's sacrifice was not pleasing to God, the smoke from his sacrifice flowed downward toward the earth.  The red glass at ground level represents Abel's blood, spilled by Cain.