Slag Glass
Slag Glass is industrial byproduct glass, (silicate glass with metal oxides). In the hand, it rates 5 to 5.5 on the Mohs scale, and it runs teal and blue. Glassy chunks in vivid swirled colors with abundant bubbles, flow streaks, and occasional metallic specks, industrial leftovers, often gorgeous.
The geology.
Slag Glass carries the formula (silicate glass with metal oxides). It belongs to the amorphous (none) system. Catalogues list it as industrial byproduct glass.
It sits at 5 to 5.5 on the Mohs scale. The surface reads vitreous. Light behaviour is transparent to opaque, often swirled. Specific gravity runs 2.4 to 2.8. The streak is white. It breaks with conchoidal fracture.
Glassy chunks in vivid swirled colors with abundant bubbles, flow streaks, and occasional metallic specks, industrial leftovers, often gorgeous.
It reaches buyers as raw glassy chunks, tumbled, 'andara' pieces or beach finds.
The origins.
Historic smelter and glassworks dumps worldwide; famously collected on beaches near old furnaces (e.g., Leland, Michigan).
On value, the database notes: affordable; regional beach slags carry charm premiums.
Traditional associations.
In energy work traditions, slag glass appears rebranded as 'andara' or galaxy glass with lofty claims, it is industrial glass with genuine local history.
We keep this section because people ask. It sits below the geology on purpose, and none of it describes something the stone does.
Spotting the real thing.
Glassy chunks in vivid swirled colors with abundant bubbles, flow streaks, and occasional metallic specks, industrial leftovers, often gorgeous.
Frequently sold as obsidian, andara crystals, or rare gems; bubbles and swirls reveal furnace origins.
| Looks like | How to tell them apart |
|---|---|
| Obsidian | Natural obsidian is black-brown family; vivid teal/purple glass is slag |
| Andara 'crystals' | Same material, different marketing |
| Fluorite | Fluorite shows crystal cleavage and is heavier |
At-home tests.
Before anyone talks you into a lab report, these are the checks worth doing yourself on slag glass.
| Test | What you should see |
|---|---|
| Under a loupe | Gas bubbles and swirl lines under a loupe |
| By eye | Conchoidal fracture everywhere |
| By eye | Sometimes metallic slag inclusions |
Stop short of anything that leaves a mark. A gemologist can answer the rest without damaging the stone.
Care & handling.
At 5 to 5.5 on the Mohs scale it handles careful wear but scratches against quartz and dust. Store it separately and take rings off before rough work.
It breaks with conchoidal fracture, which is worth knowing before you set it or drop it.
Skip the ultrasonic. Lukewarm water, mild soap and a soft cloth handle routine cleaning without risk.
A deeper look.
Extended detail on composition, sourcing, authentication and market value.
The physical record
The formula on record is (silicate glass with metal oxides). Crystal system: Amorphous (none). Specific gravity: 2.4 to 2.8. Streak: White. Cleavage and fracture: Conchoidal fracture.
How it reaches the market
Historic smelter and glassworks dumps worldwide; famously collected on beaches near old furnaces (e.g., Leland, Michigan).
What it gets confused with
Obsidian: Natural obsidian is black-brown family; vivid teal/purple glass is slag. Andara 'crystals': Same material, different marketing. Fluorite: Fluorite shows crystal cleavage and is heavier.
What it is worth
Affordable; regional beach slags carry charm premiums.
Where buyers get caught
Frequently sold as obsidian, andara crystals, or rare gems; bubbles and swirls reveal furnace origins.
One more thing
Leland Blue, Michigan's beloved beach 'gem', is pig-iron smelter slag from the 1880s, now a regional treasure.
An education-first guide.
We do not carry slag glass. This page exists because the question comes up and the honest answer is worth writing down, whether or not there is anything here to sell you.
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