Diamond Colliery

Overview

An anthracite drift at Ystradgynlais in the upper Swansea valley, opened in 1874, whose hard, bright 'diamond' coal gave it its name; it employed some 400 men in the 1920s and 1930s, and its site is now Diamond Park.

History

The Diamond Colliery lay at Ystradgynlais in the upper Swansea valley, on the northern outcrop of the coalfield, right on the old county border: its buildings and tips were in Breconshire, the entrance to its workings a few yards over the line in Glamorgan, with the River Tawe close by and the Ynyscedwyn ironworks across the river. It was a drift, opened in 1874 by the Diamond Anthracite Mine Company. Its anthracite was among the hardest of all, with a shiny black surface that left no mark on a white handkerchief; it rattled like broken china when shaken and burned fiercely with little ash.

By 1913 it belonged to the New Diamond Colliery Company of Swansea, whose sales agent, T. H. Henderson, went on to form Henderson's Welsh Anthracite Collieries. It employed 227 men in 1908, and was idle for a time from April 1915 when the government banned anthracite exports, with about 300 men giving notice in a dispute over shifts; it had 332 men in spring 1916. Diamond Anthracite Collieries Ltd owned it by 1921, the Gurnos Anthracite Collieries in 1925, and by 1930 it was part of Amalgamated Anthracite Collieries, with 400 men working seams including the Bute or Brass, 39 inches thick. In 1935 its No. 1 drift abandoned the Upper Black, Brass, Nine-Feet and Big Veins and No. 2 the Cornish and Three-Feet. Local histories give its closure as 1938; by 1945 six men underground and two on the surface were kept for pumping, and the National Coal Board closed it in April 1947, working the Four-Feet. The site was landscaped as Diamond Park, owned by Ystradgynlais Town Council, where a memorial to the district's colliers was unveiled in 2014.

Geology

South Wales anthracite coalfield, northern outcrop: the Big Vein, Nine-Feet, Upper Black, Bute or Brass (39 in), Cornish, Three-Feet and Four-Feet, all anthracite.

Workings

Drifts Nos. 1 and 2 and a return slant.

Closure

Main working abandoned in 1935; closed in 1938 by local accounts, with pumping kept up until the NCB closed it formally in April 1947.

What remains

The site is landscaped wooded parkland, Diamond Park, owned and managed by Ystradgynlais Town Council.

Redevelopment

Landscaped as Diamond Park, with a miners' memorial unveiled in 2014.

Related mines

• Gurnos Colliery — Owned by the Gurnos Anthracite Collieries in 1925, returned with Gurnos in 1933, and drawn on the same BGS plans as the Gurnos slants.

What's uncertain

Points where sources disagree or the record could not be settled; both accounts are kept. All open questions in the archive.

• The Diamond Colliery closed in 1938. (disputed) ^1,2
• The Diamond Colliery was closed by the National Coal Board in April 1947. (disputed) ³

Recorded claims

Individual factual claims, each attached to the source(s) that support it below. Where sources disagree, both are kept.

• The Diamond Colliery, a drift mine, opened in 1874. ^1,2
• Its anthracite was exceptionally hard, burned fiercely and left very little ash. ^1,2
• The anthracite export embargo brought the colliery to a temporary halt in April 1915. ²
• In 1930 it belonged to Amalgamated Anthracite Collieries and employed 400 men. ³
• In 1935 the No. 1 drift abandoned the Upper Black, Brass, Nine-Feet and Big Veins. ³
• The Diamond Colliery closed in 1938. (disputed) ^1,2
• The Diamond Colliery was closed by the National Coal Board in April 1947. (disputed) ³
• The site is now Diamond Park, owned and managed by Ystradgynlais Town Council. ²
• BGS holds plans of the Diamond Nos. 1 and 2 and return slants with the Gurnos or Wernplemis slants. ⁴

Photographs & further reading

Sources

1. (secondary) Victorian Ystradgynlais - high quality anthracite (Powys County Council local history) — Powys County Council
2. (secondary) Diamond Park, Ystradgynlais (History Points) — History Points
3. (secondary) Diamond Colliery (Northern Mine Research Society) — Northern Mine Research Society
4. (primary) BGS mine plan: Diamond Nos 1, 2 & Return slants / Gurnos or Wernplemis slants — British Geological Survey

Record created: 6 October 2026 · Last researched: 6 October 2026