Puddingstone
Puddingstone, a quartz-pebble conglomerate, is a rock in which smooth, rounded pebbles of quartz, chert, flint or jasper are locked in a hard silica cement. The name compares the pebbles to plums in a Christmas pudding.
Puddingstone, a quartz-pebble conglomerate, is a rock in which smooth, rounded pebbles of quartz, chert, flint or jasper are locked in a hard silica cement. The name compares the pebbles to plums in a Christmas pudding.
🪨 Geology Guide Specimen: Open Quartz Pebble Conglomerate (Puddingstone) in the CRV Science Rock & Mineral ID Guide to log the specimen, compare it with the other 6 subgroups of Sedimentary, and test your eye in LithoClade.
Mature Gravel: Why Is the Rock So Hard?
Puddingstone's pebbles are the most weathering-resistant minerals and rocks, mainly quartz and chert, because softer material did not survive long transport. The matrix is cemented with quartz, so the rock is as hard as its pebbles and breaks straight through them rather than around them.
Pebble Sources: Where Do Colorful Pebbles Come From?
Red and black jasper pebbles, white quartz and flint come from chert and veins in older rock. In Hertfordshire, England, the famous puddingstone has flint pebbles in a silica-cemented sand. Massachusetts has adopted Roxbury Puddingstone, a rock of the Boston Basin, as its state rock.
Uses: What Was Puddingstone Used For?
Its hardness made it a source for millstones and querns, and its striking appearance makes it a decorative stone for jewelry and ornaments. A larger family of ordinary conglomerate shares its basic structure but not its extreme toughness.
Puddingstone at a Glance
| Property | Value |
|---|---|
| Type / variety | Jasper–Quartz Clast Puddingstone |
| Composition | Rounded White Quartz & Red Jasper Pebbles in Silica Quartzite Cement |
| Geology Guide group | Sedimentary Rocks (Clastic, Chemical & Biogenic) |
| Rock Classification | Coarse Clastic (Rudites) |
| Mohs hardness | Mohs 7 |
| Texture / habit | Well-rounded pebbles (>2 mm) welded by hard silica cement |
| Density | 2.62–2.68 g/cm³ |
| Lustre and cleavage | Fractures straight through pebbles and silica cement alike |
| Geological setting | Reworked high-energy braided fluvial and marine transgressive lag deposits |
| Diagnostic feature (Geology Guide) | Distinctive gravel rock studded with smooth white quartz pebbles and bright red-and-black jasper clasts (>2 mm) locked inside a white silica cement so hard the rock breaks straight through the pebbles. |
| Pebbles | Quartz, chert, flint or jasper |
| Cement | Silica |
See Also
- Conglomerate — the broader rock family
- Quartz Arenite — a similarly mature, quartz-rich sandstone
- Sedimentary Breccia — a coarse rock with angular clasts