igneous
Slag
Anthropogenic Vitreous Silicate / Ferro-silicate Slag
Hardness: 5-7 (Mohs scale); Color: Gray, black, greenish-brown; Luster: Glassy to dull; Structure: Amorphous/Vesicular (pitted with air bubbles); Specific Gravity: 2.5-4.0
- Hardness
- 5-7 (Mohs scale)
- Color
- Gray, black, greenish-brown
- Luster
- Glassy to dull
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Physical properties
Hardness: 5-7 (Mohs scale); Color: Gray, black, greenish-brown; Luster: Glassy to dull; Structure: Amorphous/Vesicular (pitted with air bubbles); Specific Gravity: 2.5-4.0
Formation & geological history
Slag is a byproduct of metal smelting or glass manufacturing. It forms when metal is separated from its raw ore at high temperatures, creating a molten silicate waste that cools quickly. It can be centuries old or modern.
Uses & applications
Historically used for road ballast, railroad fill, or as an aggregate in concrete. Occasionally used in jewelry (e.g., Leland Blue) or as a garden decorative stone. Most common as an industrial waste product.
Geological facts
While not a natural rock, slag is often mistaken for meteorites or volcanic lava (scoria) due to its holes and dark color. Some varieties, like those from old iron refineries, are now collected as historical artifacts.
Field identification & locations
Identify by looking for air bubbles (vesicles), glassy/swirled textures, and presence near old industrial sites or railways. Unlike meteorites, slag frequently contains internal gas bubbles visible to the naked eye.
