It can have a granitic composition for the reason that it can be composed of the similar minerals like that of granite, however the magma cools right away prior to crystals having a chance to form in the rock rather than forming into a rock created with interlocking crystals.
Answer: Obsidian is an igneous rock that forms in the same way a granite forms, so their silica content is also quite same. When hot magma that are highly rich in silica and which are highly viscous because of its high silica content, comes out to the surface after eruption, it undergoes fast cooling, as a result of which the crystals do not get much time to form larger crystal. So they contain smaller crystals. And this dark variety of obsidian such as red or jet black is due to the presence of iron oxides such as hematite and magnetite.
Thus, obsidian and granite has a similar composition and both are igneous rocks.
