Sweetwater Agate Identification Guide
Identifying Sweetwater agate by its translucent grey chalcedony with black manganese dendrites, quartz hardness, and its Wyoming Sweetwater River origin.
Read the full Sweetwater Agate encyclopedia entry →
What Sweetwater Agate Looks Like
Sweetwater agate, named for the Sweetwater River area of central Wyoming, is a translucent grey-to-milky chalcedony filled with tiny black or dark manganese-oxide dendrites, fine moss-like or speckled inclusions resembling scattered black dots, sprays, or tree-like growths. The base is usually a smoky, cloudy, semi-translucent grey to nearly clear quartz that glows when held to light, with the dark dendrites suspended inside like ink in water. Pieces are typically small nodules and pebbles with a frosted exterior, revealing their interior detail only when polished or backlit.
Key Visual Cues
- Translucent grey to milky chalcedony body
- Tiny black manganese dendrites or speckled inclusions inside
- Small frosted nodules and pebbles
- Glows softly when held to light
Step-by-Step Field ID Checklist
- Hold to light. A translucent grey glow with internal black dendrites strongly suggests Sweetwater agate.
- Spot the dendrites. Look for tiny dark dots, sprays, or fern-like manganese inclusions floating within.
- Test hardness. Chalcedony is Mohs 6.5 to 7 and scratches glass.
- Check the streak. White streak.
- Inspect the fracture. Conchoidal, glassy break.
- Note size and skin. Often small frosted nodules with a translucent, waxy interior.
Diagnostic Tests
- Mohs hardness: 6.5 to 7.
- Streak: white.
- Fracture: conchoidal, no cleavage.
- Specific gravity: ~2.6.
- Acid: inert to hydrochloric acid.
- Inclusions: manganese-oxide dendrites are the diagnostic interior feature.
Common Look-Alikes and How to Tell Them Apart
- Montana moss agate: moss agate has green or brown moss-like inclusions and is often more strongly patterned; Sweetwater agate is characteristically grey-translucent with finer black dendritic speckling.
- Dendritic agate (general): Sweetwater is a regional dendritic agate; the distinguishing factor is its Wyoming origin, small nodular form, and smoky-grey translucency.
- Common grey chalcedony: plain chalcedony lacks the internal black dendrites that define Sweetwater agate.
- Snowflake obsidian: obsidian is glass (no internal dendrites, conchoidal glassy fracture, softer at ~5 to 5.5) with white cristobalite spherulites rather than black manganese dendrites.
- Plume agate: plume agate shows colorful feathery plumes; Sweetwater dendrites are typically dark and tree- or dot-like.
Where Sweetwater Agate Is Found
Sweetwater agate is found in central Wyoming, USA, in the high plains and sagebrush flats of the Sweetwater River drainage, including areas around Jeffrey City and the Granite Mountains. The agates weather out of their host rocks and are collected as surface float scattered across the ground. They form as silica-rich solutions deposit chalcedony in cavities, with manganese-bearing fluids creating the characteristic dendritic inclusions during growth.
Frequently asked questions
How can you tell if it's real Sweetwater agate?
Genuine Sweetwater agate is a translucent grey chalcedony that glows when held to light and contains tiny black manganese dendrites or speckles inside. It has a hardness of 6.5 to 7, scratches glass, leaves a white streak, and comes as small frosted nodules from central Wyoming.
What does Sweetwater agate look like?
It looks like a small, smoky grey to milky translucent pebble or nodule with fine black dendritic inclusions, dots, sprays, or tiny tree-like growths, suspended inside that show up clearly when backlit or polished.
Where is Sweetwater agate found?
It is found in central Wyoming along the Sweetwater River drainage, including areas near Jeffrey City and the Granite Mountains, where collectors pick it up as surface float in the sagebrush flats.
Sweetwater agate vs Montana moss agate, how do they differ?
Montana moss agate typically shows green or brown moss-like inclusions, while Sweetwater agate is characteristically grey-translucent with finer black manganese dendrites and comes from Wyoming as small nodules.
What causes the black dendrites in Sweetwater agate?
The dark, tree-like dendrites are manganese-oxide minerals that crystallized within the chalcedony as iron- and manganese-bearing solutions infiltrated the growing agate.
Sweetwater Agate identified by the community
Recent Sweetwater Agate specimens identified with Rock Identifier.
