Aluminum oxide is brown in color and is known as fused (brown) alumina (Al203).
Brown fused aluminum oxide is a hard, blocky shape material (9 on Mohs scale of hardness) with high wear strength.
In the sandcarving system, you should be using virgin-fused aluminum oxide manufactured to conform to military specification for the best in exact grain size and cleanliness.
Silicon carbide (SIC02) is black in color and known for its hardness and its ability to withstand high heat (Mohs hardness 9-10). It maintains its angular shape, making silicon carbide an outstanding material for sandcarving.
Either material is not dangerous but should not be inhaled. Proper safety protection is required when working with fine dust and particulate.