Silver Portoro Marble Slab

Detailed Specifications

Silver Portoro is the white-veined side of the Portoro family. The Portoro bed lies in the province of La Spezia in Liguria, Italy, and it is a black limestone of roughly 92–94% calcium carbonate rather than a metamorphic marble — the scientific literature itself writes the word "marble" in quotation marks. Within the bed, the white-veined levels lie beneath the golden ones; the horizon that separates Macchia Larga from Macchia Fine is the Nero e Bianco di La Spezia, also called Portargento, "silver port", and that is the material this page describes. The name has since travelled: Chinese 銀白龙 (Silver White Dragon) from Guangxi, and Vietnamese stone, are both sold as Silver Portoro at a fraction of the Italian price.

Two consequences follow for a specification. The veins are calcite that filled fractures in an already heavily fractured deposit, so they are planes of weakness: the stone is worked in small pieces, is not recommended for heavy foot traffic, and belongs in interiors as feature walls, vanities, furniture and inlay. And it is a carbonate stone, so it etches on contact with acid and no sealer prevents that — see the fabrication and service section below.

Specifications

Two Different Stones Are Sold as Silver Portoro — Establish Which One You Are Buying

The Portoro bed lies in the province of La Spezia in Liguria, Italy, in the municipalities of Portovenere and La Spezia — on the Castellana slopes and the Santa Croce mountains. Its history is on the islands: the quarries on Palmaria and Tino are all abandoned, and the last one on Palmaria, La Caletta, closed in 1983 because of the environmental damage quarrying was doing to the island. That ground is now the Regional Natural Park of Porto Venere, protected since 1985 and a UNESCO World Heritage Site since 1997, so nothing will be reopened there. Extraction today comes from a very small number of active quarries on the mainland, increasingly worked underground in large chambers supported by pillars precisely to limit damage to the landscape. Even the scientific literature puts the word marble in quotation marks — "Portoro, the black and gold Italian 'marble'" — because it is a black limestone, around 92–94% calcium carbonate. The number of working faces is small even by European standards, and the sources do not agree on exactly how small: a peer-reviewed survey of the deposit counted five quarries still active near La Spezia in 2015, while current trade sources put it at two or three. What is not in dispute is the direction of travel. The most recent concession award, for the Monte Santa Croce site above the city, was made in November 2023 with an annual fee payable even when the quarry is idle, and as of mid-2025 it had still not moved past the exploration-permit stage. This is not a deposit that can scale to meet demand, which is the real reason the price is what it is.

Silver Portoro is not a marketing invention, but it is also not the gold-veined stone most people picture. Within the Portoro sequence the white-veined levels sit beneath the golden ones, and the horizon that separates Macchia Larga from Macchia Fine is the Nero e Bianco di La Spezia, also called Portargento — literally "silver port". That is the material where the veins read white and silver rather than gold, and it is the level the traditional trade rates below the golden Macchia Larga. So a silver-veined Portoro is a genuine horizon of a genuine deposit, and it is also the part of the sequence that has always been the more affordable one.

The name has also migrated to China, and this is where most buyers actually end up. Chinese 银白龙 — Silver White Dragon, or Silver Dragon — is a black marble with white and silver veining quarried in Guangxi, and it is exported as Silver Portoro, China Nero Portoro and Silver White Dragon. English-language listings place the quarries around Hezhou, promoted as China's marble capital, Guilin with a finer texture, and Wuzhou in smaller quantities; Chinese supplier pages point to Xincheng county in Laibin, chiefly the Silian and Datang districts, and to Guanyang near Guilin. Xincheng in Laibin is the best-documented locality, but the sub-locality is contested, so confirm the actual mine rather than the province. Vietnamese material is also sold under the Portoro name. The gap between the Italian and the Asian stones is roughly tenfold on price, and it buys a different stone, not the same stone cheaper.

Two listing patterns are worth knowing before you ask for quotations, because both are common under this name. The first is the specification table that says "Origin: Italy" while the product description further down says the stone is sourced from Guangxi. The second is invented geology: pages offering "Italian Portoro" with the quarries placed in Tuscany, with a formation age given as billions of years, or with volcanic magma somewhere in the story. Portoro occurs in one small area — the province of La Spezia in Liguria, not Tuscany — and it is a sedimentary limestone of roughly 200 million years with no volcanic component at all. When a listing's geology is wrong, its origin claim is worth nothing, whatever the price says.

Grading Portoro-Type Stone: What the Name Promises and What It Does Not

The Portoro deposit is stratified, and the traditional grading follows the bed from top to bottom. The Scalino is the first quality: absolute black carrying at least five golden-yellow veins, in a layer 0.8–1 m thick. Below it the Banco shows at least seven veins, some wider, in 1.2–1.5 m of stone. Then come the Sottobanco and Zoccolo, where the yellow veins narrow and diffused white veins begin to appear, and finally the Sottozoccolo, thickest and whitest and the least valuable. A dolomite-rich facies, the Tarso, is not commercialised at all. Commercially the trade then sorts Macchia Larga and Macchia Fine into four qualities each — eight typologies in total, per Cimmino et al. (2003) — and not all eight are still available on the market. Portoro Extra is the top of the tree: the densest and cleanest, a deep uniform black with no white streaks and continuous gold veining running across the whole slab.

Two conclusions follow for a buyer. First, "Silver Portoro" describes a position in that sequence — the whiter, cheaper end — not a grade, so the name alone tells you nothing about whether the slab in front of you is a clean, uniform, well-veined piece or a grey, cloudy one. Second, the veins are the axis of value in both directions: in the golden grades buyers pay for more gold, in the silver grades they pay for more contrast and less cloudiness. Specify the vein character you want — bold lightning-bolt runs, a fine mesh, or soft clouding — because those are different products at different prices from the same bed.

Vein density is not only a look. These veins are calcite that filled fractures, so they are planes of weakness in a rock that is already intensely fractured by the tectonics that created it. That is why Portoro-type stone is sold in small pieces, why block recovery is low, and why the trade advises against using it in heavy foot-traffic areas: it is an interior stone for feature work, vanities, furniture and inlay, not a shopping-centre floor. The size constraint is real and worth designing around. Italian Portoro blocks are described in the trade as small to medium, with heavy fracturing reported to limit many pieces to around a cubic metre, and the Italian slab trade has settled accordingly on 20 mm thickness and formats smaller than the marble norm. Design to the format that exists rather than the one you would prefer, and check the maximum slab size in writing before you fix a wall layout.

Fabrication, Finish and Performance in Service

Treat this stone as delicate and specify accordingly. It is a carbonate rock, so it etches on contact with acid — wine, vinegar, citrus, tomato and acidic bathroom cleaners all leave dull marks, and a penetrating sealer slows staining but does not stop the chemical reaction that removes the polish. On a black ground the etch shows as a pale, milky patch, more visible than it would be on a light marble, and honing does not hide it the way it hides etches on white stone, though the effect is far less glaring than on a polished face. The practical regime is pH-neutral cleaning, resealing about every 12 months in humid or coastal situations and every 18–24 months inland, no hydrogen-peroxide poultices on a dark stone, and honed or leathered finishes anywhere that will be touched, walked on or cleaned regularly. An etch cannot be polished out chemically; it has to be re-honed with diamond abrasives, in place, which is expensive and disruptive, and a specialist restorer has documented re-polishing the same wet bar twice inside about two years. Treat a bar top or a kitchen worktop in this material as an ongoing maintenance commitment rather than a one-off purchase. See Stone Finishes Compared and the Stone Sealing Guide.

Fabrication has its own rules. Because the calcite veins are the weak planes, edges chip and mitres open unless the fabricator works in light passes and hones wet rather than dry — dry honing gouges the softer vein material and leaves the veins sitting below the polished face. Slabs and thin tiles of this type are normally backed with resin and fibreglass mesh, which is standard rather than a defect, but the mesh raises an installation question: adhesive bonds to the backing rather than to the stone, mesh-backed tiles have been documented debonding at that interface, and wet-set work may require the mesh to be removed unless it has been tested for cement compatibility. BS 5385: Part 2 rules resin mesh-backed stone out for external cladding altogether. For vanities and worktops, specify 20 mm rather than 10–12 mm, and reinforce the edges — the vein planes are where a countertop fails.

Bookmatching is the classic use of this stone and it should be planned at the block, not in the showroom. A book-matched wall needs consecutive A/B slabs cut from the same block and shipped in sequence, and it needs the seam positions and the centre line fixed at wall scale before the slabs are cut. Keep the joint tight and the grout colour matched; a wide grey joint destroys the mirror. Dry-lay and photograph the assembly before packing, and approve that photograph rather than a catalogue image.

Buying Silver Portoro Slabs: What to Verify, and What It Should Cost

Origin first, then grade, then the physical inspection. Ask for the quarry and the province in writing, an origin certificate for Italian material, the vein character and background blackness matched to a sample you have physically approved, the finish and thickness with the calibre tolerance stated as ±1 mm commercial or ±0.5 mm premium, sequential slab numbers within a reserved block lot, and a batch test report for water absorption and flexural strength on the material actually loaded. Approve the bookmatch or dry-lay before packing, and inspect before the container is sealed. Order 5–10% overage, which is normal practice on this material, and at the top of that range if any edge is mitred or radiused, because low flexural strength and calcite vein lines make chipping the main fabrication loss. Name the grade the way the Italian trade does — macchia fine, mezza macchia, macchia grossa — rather than "premium": those words come off the quarry bench, and a supplier who cannot discuss macchia grading is probably not buying at the face. And run an acetone wipe on a hidden corner of each bundle; if the rag pulls black, the material has been dyed, which is common on black carbonates and is a rejection item rather than a cosmetic one.

On price, published figures diverge by an order of magnitude because the name covers different rocks. Genuine Italian Portoro is one of the most expensive stones traded. One Italian regional public-works price list (Piemonte, 2026) prices Portoro slabs in the mezza macchia grade at about €308 per m² for 2 cm rising to about €1,168 per m² at 10 cm — bare rates, before the overhead and safety percentages such lists add on top, and a public-works reference rather than a dealer quotation. Trade listings put white- and grey-veined material such as Portargento at roughly €450–750 per m² against €750–1,500 per m² for the finest gold-veined top layer, and a UK retailer has listed a single Nero Portoro Gold slab at £1,680 per m² including VAT. Mid-range Italian material is reported at around €650 per m² ex-factory. Real Italian stock moves in small batches — a UK supplier's lots have been listed as 12 slabs totalling about 70 m² and 22 slabs totalling about 98 m². Chinese 银白龙 exports as Silver Portoro with reference prices around US$17–20 per m² FOB Xiamen at a minimum order of 100 m², and Chinese domestic factory-direct slab prices for the same material are published at roughly 55–95 RMB per m². An Indian listing for "Silver Portoro" starts around US$42 per m². Treat each as a reference price from a listing rather than a negotiated quotation, and remember that what sets your price is origin, vein character, slab size, calibre, finish and whether a specific block lot is reserved. Shipping to the United States, price the 25% Section 301 duty on Chinese stone products into the landed cost. The Marble Price Guide explains how to read a quote structured this way and Negotiating with Stone Suppliers covers the commercial levers.