How mineral auctions work
How a mineral auction works for a specimen or a collection: consignment, reserve, the hammer, buyer’s premium on 100 units, 5 to 7 days to pay, 45 to settle.
A mineral auction runs in a fixed order: estimate, consignment with a reserve, catalogue, bidding and the hammer, which completes the sale under the Sale of Goods Act 1979; then payment and settlement. On a hammer of 100 units the buyer pays 120 to 130 before other taxes under the terms we read; the seller waits up to 45 days.
In short
- The hammer is the contract. Section 57(2) of the Sale of Goods Act 1979 says a sale by auction is complete when the auctioneer announces it by the fall of the hammer, and any bidder may retract a bid until then. Sotheby’s London and Spink’s published terms say the same in their own words.
- Buyer’s premium, worked on 100 units of hammer. Sotheby’s London conditions of 2 July 2024: 20 premium, 120 before taxes. Sotheby’s help page, updated 9 September 2026: 28 on the first band, 128. Spink’s jewellery terms: 25 premium and 20% VAT on it for a margin-scheme lot, 130.
- Documented change. The same house’s first-band rate went from 20% in its dated 2024 conditions to 28% on its 2026 help page; the 2024 document itself says the rate is subject to change at any time. Read the terms for the sale you bid in, not a remembered figure.
- The divide-by rule. To stay inside an all-in budget, divide it by 1 plus the premium rate (and any VAT on the premium); do not bid the budget. A 1,000-unit budget allows a hammer of 781 at 28%, 833 at 20% and 769 at 25% plus VAT.
- The longest published wait is the seller’s. Sotheby’s says it sends the seller’s proceeds within 45 days of the sale, provided it has received the buyer’s payment. Buyers pay within 5 days (Sotheby’s London) or 7 (Spink). A buyer who does not pay is what makes settlement slip.
| Stage | Who acts | Published timing | Source | What it means on a hammer of 100 units |
|---|---|---|---|---|
| 1. Estimate request | Seller | Preliminary estimates in 15 to 20 days | Sotheby’s guide | No money moves |
| 2. Consignment contract | Seller and house | Not published | Sotheby’s guide: the seller’s contract sets the reserve and the seller’s commission | Commission rate is in the contract, not on the fetched pages |
| 3. Catalogue and viewing | House | Not published | — | — |
| 4. Bidding | Bidders; auctioneer may bid for the seller up to the reserve | Sale day (day 0) | Sotheby’s 7(d); Spink 4.9; SoGA s.57(3)–(6) | Bidding opens below (Spink) or at or below (Sotheby’s timed) the low estimate |
| 5. Hammer | Auctioneer | Day 0 | SoGA s.57(2); Sotheby’s 7(g); Spink 4.12.1 | Contract formed at 100; below the reserve there is no sale |
| 6. Buyer pays | Buyer | Within 5 days (Sotheby’s London); within 7 days (Spink) | Sotheby’s 9(a); Spink 5.4.1 | 120 (20%), 128 (28%) or 130 (25% plus 5 VAT on the premium) |
| 7. Buyer collects | Buyer | No later than 30 calendar days (Sotheby’s London) | Sotheby’s 10(i) | Late-collection charges apply after that, per the conditions |
| 8. Seller is paid | House | Within 45 days, provided the buyer has paid (Sotheby’s) | Sotheby’s guide | 100 less the seller’s commission and agreed expenses |
What happens when I consign minerals to an auction?
Estimate first, then a contract that sets the reserve. Sotheby’s buying and selling guide says its specialists give preliminary estimates in 15 to 20 days, and that the seller’s contract sets the reserve price and the seller’s commission. Its London conditions define the reserve as the confidential minimum Hammer Price at which an auction Lot can be sold and say it cannot exceed the Lot’s low estimate. Spink’s terms define it as the amount below which the lot cannot be sold.
What the reserve does in the room. Both houses’ terms let the auctioneer bid on the seller’s behalf up to the reserve. The law allows that only when it is declared: section 57 of the Sale of Goods Act 1979 says a sale may be notified as subject to a reserve and to a right to bid by the seller, and that without such notice it is not lawful for the seller to bid. If bidding stops below the reserve, Sotheby’s guide says the piece is not sold.
What we could not find. None of the fetched pages states a seller’s commission rate or a lead time from consignment to sale day, so this page gives neither. For a mineral specimen the consignment is also where the label and the locality travel with it; see buying old collection material. The other routes for a whole mineral collection are compared on where to sell a mineral collection.
When is an auction sale final? The fall of the hammer
Under English law the sale is complete when the hammer falls, and not before. Section 57(2) of the Sale of Goods Act 1979: a sale by auction is complete when the auctioneer announces its completion by the fall of the hammer, or in other customary manner; and until the announcement is made any bidder may retract his bid. Section 57(1) makes each lot a separate contract.
The houses write the same rule into their terms. Sotheby’s London conditions say the sale contract is concluded on the striking of the auctioneer’s hammer in a live auction. Spink’s say the striking of the hammer marks acceptance of the highest bid above any reserve and the conclusion of a contract between buyer and seller. Its hammer price is the amount of the highest bid accepted by the auctioneer.
Online sales differ in one detail. In a Sotheby’s timed auction, a bid in the last minute extends that lot’s closing by two minutes from the last bid. Spink’s timed auctions close on the highest bid at the closing time. The contract still forms at the close, not at payment.
How much is the buyer’s premium? Worked on a hammer of 100 units
On 100 units of hammer the buyer paid 120, 128 or 130 before other taxes, depending on which published terms applied. The figures are nominal units, not a price for any mineral.
- Sotheby’s London conditions for buyers, 2 July 2024: premium 20% of the hammer on the first band. 100 + 20 = 120, plus any applicable VAT on the premium. The premium is 16.7% of what the buyer pays.
- Sotheby’s help page, updated 9 September 2026: 28% on the first band, then 22% and 15% on higher bands, in every sale location it lists except wine and spirits, which it charges at 24% throughout. 100 + 28 = 128.
- Spink, terms for buyers at jewellery sales: premium 25% of the hammer; on a margin-scheme lot, VAT at 20% on the premium only. 100 + 25 + 5 = 130.
Documented change: the 2024 figure is no longer the 2026 one. The same house moved its first band from 20% to 28%: 8 more units per 100, or 6.7% more on the buyer’s total. Its 2024 conditions already warn that the premium rate is subject to change at any time. Check the date on the terms of the sale you are bidding in.
How do I set a maximum bid? The divide-by rule
Divide the all-in budget by 1 plus the premium rate; do not bid the budget and do not subtract the percentage. With a budget of 1,000 units at a 28% premium, the highest hammer is 1,000 ÷ 1.28 = 781. Bidding 1,000 costs 1,280. Subtracting 28% of the budget gives 720, which leaves 61 units of hammer unused.
- Premium 20%, no VAT on it: 1,000 ÷ 1.20 = 833.
- Premium 25% plus VAT at 20% on the premium: the multiplier is 1 + 0.25 × 1.20 = 1.30; 1,000 ÷ 1.30 = 769.
- Premium 28%: 1,000 ÷ 1.28 = 781.
Where the rule needs more. Shipping, card fees, import VAT and duty are charged on top in Spink’s terms, and Sotheby’s purchase price includes any applicable taxes; add those to the multiplier if they apply. What a specimen ought to fetch is outside this page, which makes no claim about value; how to value a mineral collection for insurance covers that separately.
How long does it take to get paid after a mineral auction?
Up to 45 days at Sotheby’s, and only once the buyer has paid: that is the longest stage with a published figure. Its guide says it sends payment within 45 days from the sale date, provided we are in receipt of the buyer’s payment, and that the payment is the hammer price less a commission and any agreed expenses.
The buyer’s side is shorter. Sotheby’s London conditions require payment in cleared funds within five days of the auction and collection within 30 calendar days. Spink’s require payment within seven days of the sale, and its terms set out further steps for late payment. Title does not pass to a Spink buyer until everything due is paid.
What makes it slip. Settlement waits on the buyer’s payment, and an export licence can hold it too: Spink says it holds proceeds until any export licence is finalised. A specimen that fails its reserve is not sold at all. What to expect when buying in person is on what to expect at a mineral show; the other collecting guides are in the collecting guides. Nothing here is offered for sale; the wanted list is the only commercial route.