Ferro-slag (Leland Blue/Slag Glass)

Physical Properties

Hardness: 5.5-6.5; Color: Blue-grey, white, and purple swirls; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific Gravity: 2.5-2.8

Formation & Geological History

Formed as a byproduct of the iron smelting industry during the 19th and early 20th centuries. It is created when raw iron ore is heated and the impurities (flux) separate into a molten glass-like state. While technically man-made, it undergoes a cooling process similar to obsidian.

Uses & Applications

Used primarily in jewelry making (lapidary), as decorative landscaping stones, and as historical collectibles. Historically, it was a waste product often used as road bed filler.

Geological Facts

This material is often called 'Leland Blue' if found in Michigan, or 'Sieber Agate' in other regions. Despite being industrial waste, it is highly prized for its unique aesthetic and historical connection to the industrial revolution.

Field Identification & Locations

Identify by its glassy texture, conchoidal (shell-like) fractures, and frequent presence of air bubbles (vesicles) on the surface or interior. Commonly found near old iron foundry sites or on beaches in the Great Lakes region.

Identified on: 4/19/2026

Mode: Standard