In reality, obsidian is easily broken by human action and can also be worn down by the weather. This would make it a terrible substance from which to build a fortress. While obsidian is not used to build indestructible walls or portals, it is used to make surgical tools.

20 million years

obsidian, igneous rock occurring as a natural glass formed by the rapid cooling of viscous lava from volcanoes. Obsidian is extremely rich in silica (about 65 to 80 percent), is low in water, and has a chemical composition similar to rhyolite. Obsidian has a glassy lustre and is slightly harder than window glass.

Obsidian can be found in locations which have experienced rhyolitic eruptions. It can be found in Argentina, Armenia, Azerbaijan, Australia, Canada, Chile, Georgia, Greece, El Salvador, Guatemala, Iceland, Italy, Japan, Kenya, Mexico, New Zealand, Papua New Guinea, Peru, Scotland, Turkey and the United States.

Pure obsidian is usually dark in appearance, though the color varies depending on the presence of impurities. Iron and magnesium typically give the obsidian a dark brown to black color. Very few samples are nearly colorless. These bubbles can produce interesting effects such as a golden sheen (sheen obsidian).

So, back to the question, just how strong is obsidian in real life? Obsidian is about half as strong as diamonds, 5.0 on the Mohs scale… so not really all that strong. However, it has almost no crystalline structure. This means that it can have a really fine edge, which makes it really, really sharp…

It’s the fragmentation that makes it deadly as you have many small pieces. As noted in other answers, an obsidian bullet would basically be an untempered glass bullet and it would require quite a bit of careful machining to get it into the bullet shape.

The use of obsidian in jewelry can be limited by its durability. It has a hardness of about 5.5 which makes it easy to scratch. It also lacks toughness and is easily broken or chipped upon impact.

Obsidian is hard, brittle, and amorphous; it therefore fractures with sharp edges. In the past, it was used to manufacture cutting and piercing tools, and it has been used experimentally as surgical scalpel blades….

Scientists Create a Near-Living Crystal. Three billion years after inanimate chemistry first became animate life, a newly synthesized laboratory compound is behaving in uncannily lifelike ways. The particles aren’t truly alive — but they’re not far off, either.

Obsidian is an “extrusive” rock, which means it is made from magma that erupted out of a volcano. If it was an igneous rock that formed from magma underground and did not erupt, it would have been called an “intrusive” rock.

The magnetic properties of volcanic glass and obsidian have long been known. Selected magnetic properties have proven useful discriminators among eastern Mediterranean obsidian sources.

In a widely accepted silica-content classification scheme, rocks with more than 65 percent silica are called felsic; those with between 55 and 65 percent silica are intermediate; those with between 45 and 55 percent silica are mafic; and those with less than 45 percent are ultramafic.