What Is Black Diamond Mining? Carbonado vs Coal

Black diamond mining means one of three things, and which one you want decides everything else on this page. It can mean recovering carbonado, a rare opaque natural diamond that Gems & Gemology says is found only in Brazil and the Central African Republic (GIA, Gems & Gemology, Summer 2017). It can mean coal, which miners and writers have long nicknamed “black diamonds” (Huber Breaker Preservation Society). Or it can mean a place: the Black Diamond Mines Regional Preserve in California, a former coal and sand mining district (East Bay Regional Park District).

This guide covers all three, in the order people search for them. It gives each a short definition, the numbers behind it with source and date, a chart, and a drill-bit cost calculator for readers who buy diamond tooling. Mining terms are explained the first time they appear.

What black diamond mining means: carbonado, coal and place names

The phrase has no single legal or industry definition. The table sorts the three meanings by what is dug, where, and what it is used for. Each row is expanded in its own section below.

A fourth use of the words is jewellery. Black gem diamonds sold in shops are usually coloured by inclusions or by treatment, and they are not carbonado. That is covered near the end of this page.

Carbonado: the black diamond that comes from gravel

Carbonado is a natural diamond, but it does not look or behave like the single crystals found in kimberlite pipes (the volcanic rock bodies that carry most gem diamonds up from the mantle). It is an aggregate of tiny randomly oriented diamond crystals, it is opaque, and it is full of pores. The GIA review describes the pieces as strongly bonded and porous, with glassy, melt-like skins (Haggerty, Gems & Gemology, 2017). Portuguese prospectors in Brazil named it for its likeness to charcoal after its discovery in 1841.

The same review says its toughness comes from the random orientation of the tiny crystals, that it is clearly tougher than single-crystal gem diamond, and that carbonado can be cut only by lasers. It also notes that hardness is similar to other diamond, with some data suggesting carbonado is slightly harder. No source we checked gives a “times tougher” multiplier, so treat any such number as unsupported.

Where carbonado is found

Carbonado occurs in two regions. In Brazil the host rock is the Tombodor conglomerate in the Chapada Diamantina of Bahia, and artisanal miners have worked the stream gravels there. The other region is the Central African Republic. The GIA review notes that the two areas sat together on the same ancient continental block for over a billion years before the Atlantic Ocean opened. The review also records that no primary host rock, meaning the original source rock, has yet been identified for carbonado, nearly two centuries after its discovery.

Some history shows the scale. At the height of alluvial mining in Brazil, between 1850 and 1870, about 70,000 carats were produced by an estimated 30,000 artisanal miners. A conservative estimate for all production from both countries is about 2 metric tons in total. Four of the five largest diamonds reported from Brazil are carbonado, the largest being the Sergio of 1905, which the review says is 61 carats heavier than the 3,106 carat Cullinan rough (GIA, 2017).

Where carbonado comes from: the origin debate

Nobody has settled how carbonado formed. The GIA review sorts the proposed origins into five groups: meteorite impact, growth and sintering in the crust or mantle, subduction (rock pushed down into the mantle at a plate boundary), radioactive ion implantation, and an extraterrestrial source. It says none is uniformly accepted. A 2006 infrared study by Garai, Haggerty, Rekhi and Chance found mostly single nitrogen impurities and hydrogen in the spectra, and the authors concluded that carbonado formed in a hydrogen-rich interstellar environment, in a fragmented body of asteroidal size that they put at 2.6 to 3.8 billion years old (arXiv preprint). The US National Science Foundation release carried by Brookhaven National Laboratory, where the infrared work was done, presented the same conclusion in January 2007 (Brookhaven National Laboratory). That is the authors’ interpretation, and the 2017 review still describes the models as ranging from terrestrial subduction to cosmic sources.

For a mining reader the origin question matters for one reason: with no known source rock, there is no pipe to drill. Carbonado is found by working the gravels and conglomerates where it has already been deposited. The USGS model for diamond placers (deposits formed by water sorting heavy minerals into gravel) lists carbonado among the polycrystalline diamond types and says diamonds collect in the low-energy parts of stream systems along with other heavy minerals (USGS Bulletin 1693).

How carbonado is mined and recovered

The documented carbonado workings are small and alluvial. The GIA review shows artisanal digging of stream gravel in Bahia and a conglomerate boulder in a streambed as the host rock. There is no published modern plant description for carbonado, so the steps below are the general sequence for alluvial diamond ground, set out in order. Specific plant designs vary by site.

1. Locate the gravel. Explorers map the stream terraces and conglomerate beds, because the USGS placer model says diamonds sit in the low-energy parts of a stream with other heavy minerals.
2. Dig and break. Loose gravel is dug by hand or machine; cemented conglomerate has to be broken down first.
3. Wash and screen. Clay and fines are washed away, and the gravel is sized so that heavy material can be concentrated.
4. Concentrate by density. Heavy minerals are separated from the lighter gravel, using jigs, sluices or dense media in larger operations.
5. Hand sort and grade. Stones are picked from the concentrate by eye. Any dark stone needs a laboratory diamond test before it is sold as carbonado.

Because carbonado has not been traced to a bedrock source, there is no underground diamond mine for it. Anyone who sells “underground black diamond mining” with carbonado in mind is describing something that the sources do not show. Underground “black diamond” operations are coal mines, covered below.

What carbonado and industrial diamond are used for

Carbonado was valued for work rather than for beauty. The GIA review says the French prized it as a superior polishing material, that it was used for drilling during construction of the Panama Canal, and that it was part of the US strategic mineral stockpile as recently as 1990. The US customs tariff still lists carbonados as their own line, “Industrial Miners’ diamonds: carbonados”, separate from other industrial stones (USGS Mineral Commodity Summaries 2026, diamond (industrial)).

Chart

US production of manufactured industrial diamond

Million carats of bort, grit, dust and powder

The modern industrial market has moved on. USGS says synthetic diamond makes up more than 99% of global industrial diamond production and consumption, that natural diamond is less than 1% of industrial diamond used, and that most natural industrial diamond is a by-product of mining gem-quality diamond (USGS, February 2026). The five biggest producers of natural industrial diamond mined 95% of the world’s 38 million carats in 2025, and neither Brazil nor the Central African Republic is among them.

A carat is 0.2 gram, so 38 million carats is about 7.6 tonnes of natural industrial diamond for the whole world in a year, against more than 15 billion carats of manufactured diamond (about 3,000 tonnes).

That gap explains why tools that once used carbonado now use manufactured grit. In the United States, USGS estimates that about 97% of apparent industrial diamond consumption in 2025 was synthetic, and that the construction sector, especially highway cutting and repair, is a strong source of demand (USGS, February 2026).

Natural industrial diamond by country and the synthetic shift

The first chart shows where natural industrial diamond is mined. It is a by-product stream from gem mines in Russia, Congo (Kinshasa), Botswana, Zimbabwe and South Africa, not from carbonado ground. The second chart shows the manufactured output that fills most tool demand in the United States.

Chart

World natural industrial diamond mine production

Million carats, 2025 estimate, selected countries

Black diamond mining for coal: anthracite and underground mines

If your search was for “underground black diamond mining” or “which mineral is known as black diamond”, the answer is usually coal, and most often anthracite. The Huber Breaker Preservation Society, a heritage group in Pennsylvania’s anthracite region, describes anthracite as the shiniest coal, often called “black diamonds”. The US Energy Information Administration (EIA) explains the four ranks of coal: anthracite (86% to 97% carbon), bituminous (45% to 86%), subbituminous (35% to 45%) and lignite (25% to 35%). Anthracite made up less than 1% of US coal mined in 2022, and all US anthracite mines are in northeastern Pennsylvania (EIA, Coal explained).

Chart

US coal production by state, 2024

Million short tons, the eight largest producing states in the report

The EIA Annual Coal Report for 2024, released in November 2025, gives the scale. US coal production fell 11.3% to 512.5 million short tons (a short ton is 2,000 pounds, about 0.907 tonne), from 524 mines. West Virginia produced 79.5 million short tons and Pennsylvania 40.6 million, against 190.7 million from Wyoming, where all coal is surface mined (EIA Annual Coal Report 2024). Adding the three regional underground totals in the same report (Appalachia 125.7 million, Interior 58.8 million, Western 21.7 million short tons) gives about 206 million short tons from underground mines, roughly 40% of the total. That share is our calculation from EIA’s table, not a figure EIA states.

Underground coal is the expensive end of the business. EIA reports an average 2024 sales price of $85.88 per short ton for coal from underground mines against $29.61 for surface mines, with productivity of 3.24 short tons per employee hour underground against 8.94 at the surface. Metallurgical coal, the grade used in steelmaking, averaged $180.02 per short ton in 2024 against $37.85 for thermal coal. By rank, anthracite averaged $156.21 per short ton, bituminous $86.72, lignite $22.81 and subbituminous $18.06 (EIA, 2024 data).

Every underground mine adds hazards that open pits do not, including roof falls, gas and dust. For the safety systems used on such sites, see our guide on mining safety systems ranked by hazard, and for the land and soil effects of coal mining see our note on coal mine impacts on land and soil.

Black Diamond Mines in California: coal first, then sand

Several searches for “black diamond mine” or “black diamond mine road” are about places, not minerals. The best known is the Black Diamond Mines Regional Preserve near Antioch in Contra Costa County, California. California State Parks lists the Mount Diablo Coal Field there as the largest coal-mining district in the state from 1860 to 1914, with five towns (Nortonville, Somersville, Stewartville, Judsonville and West Hartley) near twelve major mines (California State Parks, Office of Historic Preservation).

The East Bay Regional Park District says nearly four million tons of coal, the “black diamonds” of the park’s name, were removed from this ground. Coal mining stopped because of rising production costs and new energy sources. Underground sand mining began in the 1920s, supplying glass and foundry sand, and ended by 1949 after more than 1.8 million tons of sand had been mined. The District began acquiring land for the preserve in the early 1970s, and the mining district now sits mostly inside about 8,533 acres of parkland (East Bay Regional Park District).

If you are planning a visit, check the District’s page for current hours, fees and tour rules, since those change.

▶ A plain-language claymation on the seven environmental impacts of mining. · 2:31

Diamond tool cost per metre drilled

Whatever the label on the stone, buyers of diamond drilling and cutting tools compare cost per unit of work, not the price of the bit. The calculator below turns a bit price, its life, and your rig cost into a cost per metre. It is a simple model: it ignores rod handling, water, tripping time and ground variation, which differ widely between sites. For companies and permits in diamond core drilling, see our diamond drilling guide for Canada.

How to tell carbonado from a treated black diamond

Most “black diamond” jewellery is not carbonado. GIA research on black diamonds from Marange in Zimbabwe notes that natural black diamonds are usually coloured by inclusions of sulfides, graphite, magnetite, hematite or other iron-bearing minerals. Treated stones are often heavily fractured diamonds that are heated at low pressure and high temperature to turn the fractures to graphite, which makes them black. The same paper states that the challenge for gem laboratories is to separate treated black diamonds from natural ones with confidence (GIA, Gems & Gemology, Summer 2018).

The practical rules for a buyer or a field geologist follow from those sources:

• Carbonado is opaque and porous with a glassy skin; a smooth, polished black gem is a different material.
• Only a laboratory test can separate natural, treated and synthetic black stones. A scratch or drop test on a sample is not proof, and can damage it.
• Check the origin. If a seller claims carbonado from anywhere other than Brazil or the Central African Republic, ask for the laboratory report.
• Check export rules. Diamond exports from many countries need Kimberley Process certification, so ask for the certificate with any parcel. Details are on the Kimberley Process website.

Can satellites help find carbonado ground?

Satellites cannot see a diamond of any kind. What they can map is the setting: river terraces, drainage patterns, conglomerate outcrops, and surface colour and mineral clues in the rock, which an explorer can use to rank where to send a field team first. For alluvial ground the useful clues are the old and present stream systems that could have carried and trapped heavy minerals. A satellite screen of this type produces exploration targets that sit ahead of any resource estimate, and every target still needs ground checks, sampling and local permission.

Farmonaut works with licence owners, consultants and investors who want that first screen of a licence area from space. You can read more about the satellite-based mineral detection service, send a polygon through the mining query form, or contact the team.

▶ Explains remote sensing for mining in about two minutes. · 2:01

Frequently asked questions

What is black diamond mining?

It is a phrase with three common meanings: recovering carbonado, a rare opaque diamond found in Brazil and the Central African Republic; mining coal, nicknamed “black diamonds”; or the history of the Black Diamond Mines district in California. Carbonado is the diamond meaning; coal is the largest by tonnage.

Which mineral is known as black diamond?

Two minerals carry the name. Carbonado is a black polycrystalline form of diamond. Anthracite, the hardest and shiniest coal, is also called “black diamonds”, as the Huber Breaker Preservation Society explains for Pennsylvania.

Where is carbonado mined?

Only in Brazil and the Central African Republic, according to the GIA review of 2017. The Brazilian site it documents is Bahia. In Brazil the host rock is the Tombodor conglomerate and the work is mostly artisanal alluvial digging.

How is black diamond mining done?

For carbonado it is alluvial: gravel is dug, washed, screened, concentrated by density and hand sorted. For coal it is either surface mining or underground mining, and about 40% of US coal in 2024 came from underground mines by our calculation from EIA’s table.

Is there underground black diamond mining?

Not for carbonado, which has no known bedrock source. Underground black diamond mining refers to coal, such as the underground anthracite and bituminous mines of Pennsylvania and West Virginia.

What is black diamond mining used for?

Carbonado was used for polishing and for drilling, including on the Panama Canal. Coal is used for electricity and steelmaking. Today more than 99% of industrial diamond is synthetic, according to USGS.

What is the black diamond mining process for carbonado step by step?

Find the gravel or conglomerate, dig and break it, wash and screen it, concentrate heavy minerals, then hand sort and test the stones. The steps are the general alluvial diamond sequence; no carbonado-specific plant is described in the sources.

How much is a black diamond worth?

There is no single price. Gem-quality treated black stones, natural carbonado specimens and industrial grit sell in different markets, and USGS estimates the 2025 unit value of imported industrial diamond stones (natural and synthetic) at $6.70 per carat. Ask for a laboratory report and a current quote for the exact material.

Is black mining the same as black diamond mining?

Not necessarily. No source we checked defines “black mining”, so check the context before you trust a figure. This page covers carbonado, coal and the California Black Diamond Mines district.

What is carbonado?

A porous, opaque, polycrystalline diamond named by Portuguese prospectors for its resemblance to charcoal. Its origin is still debated, with proposals ranging from subduction to cosmic sources.

Is carbonado from space?

One 2006 infrared study concluded that it formed in a hydrogen-rich interstellar environment. That is the authors’ conclusion, and other origins are still discussed in the 2017 GIA review.

Where is the Black Diamond Mines Regional Preserve?

Near Antioch in Contra Costa County, California, at 5175 Somersville Road. It covers about 8,533 acres and includes the former Mount Diablo Coal Field and its sand mines.