TumbleStone Calendar 2026: November – Breccia

The Introduction to this Calendar can be found here. 

The month of November in the 2026 TumbleStone Calendar features four breccia stones. A breccia stone is made up of angular non-rounded fragments, sometimes from different sources, cemented together in a fine-grained matrix. The matrix may be a mineral like quartz or calcite, or a material like silt or clay (in a petrified state). As University of Auckland Geology notes, if the fragments (also known as “clasts”) are rounded, the stone is instead called a conglomerate. Water is the most common cause of such rounding. Sometimes in a breccia, the fragments come from a single rock or rocks of the same type – these are called “monomictic” (“mictic” meaning “mixed”). Where the fragments comes from two or more rocks types, the stone is called “polymictic”. One of the main ways that breccia forms is when material accumulated on steep hill slopes or at the foot of cliffs consolidates into new rock under pressure and heat. Some breccias are produced by the fragmentation of rocks during faulting or tectonic stress, and volcanic breccias comprise small blocks of lava in an ash matrix, the product of a volcanic eruption. Two less common formation processes are when hydrothermal fluid fractures a rock mass, and when a meteor fractures rock at the site of an impact. Sandatlas provides a good detailed account of breccia and its formation.

Stone One (top row, left, in Calendar) is a monomictic breccia from Ward Beach, Marlborough, found on 2 April 2025:

It’s likely to be limestone, a common rock in this area – it looks like the rock has fragmented under tectonic pressure and then heated water flowing through it has left something like quartz behind to fill the gaps.

Stone Two (top row, right, in Calendar), found on Gemstone Beach on 15 March 2025, is very unusual as it has a brecciated vein:

This monomistic breccia stone is very similar to the one featuring in the Post “T is for a Type of Tiny Tectonic Breccia?” (not yet published), a green-hued stone with a vein containing tiny fragments of the same stone. As I note there, “Definitions of breccia usually refer to large-scale masses of rock, and they do not cover this form of brecciation WITHIN a small stone.”

Another green breccia is Stone Three (bottom row, left, in Calendar), found on Gemstone Beach on 27 March 2025:

The green matrix is probably the mineral epidote which is usually described as “pistachio green”. The light-coloured fragments or clasts all look like they might be a fine-grained mudstone, again making this a monomictic breccia.

Stone Four (bottom row, right, in Calendar), found on Slope Point on 10 March 2025, is polymictic, its clasts coming from different sources:

The fragments don’t stand out as clearly as in Stone Three but careful examination shows a range of them in terms of size, colour and make-up. Particularly attractive are the lightest coloured ones with tiny spots in them. These spots might be crystals or even tiny fragments – if the latter, this would mean that some of Stone Four’s clasts are themselves originally part of a different breccia.

For December’s hydrogrossular garnets, see this Post.