2018-S Block Island, Silver Proof

San Francisco closed the 90%-silver ATB run with Phebe Hemphill's Block Island design at 451,510 pieces, the figure matching the U.S. Mint's Silver Proof Set sales total for 2018. The reverse pairs a black-crowned night heron in flight with an American oystercatcher and chick on the wrack-line beach, both species the wildlife refuge on the Rhode Island offshore island was established to protect when Congress designated Block Island National Wildlife Refuge in 1973. The composition is the 90% silver, 10% copper alloy at 6.25 grams with .1808 troy ounces of pure silver, the alloy the Mint had used for silver proof quarters since 1992. 2018 is the final year of the .900 fine specification: beginning in 2019 the Mint converted all silver proof denominations (dimes, quarters, half dollars) to .999 fine silver, making the Block Island issue the closing 90%-silver ATB strike and the literal last 90%-silver quarter the U.S. Mint produced. The issue is the fifth and final 2018 silver release and the 45th of the 56-design ATB program.

Authentication on the issue starts with weight. The 6.25-gram silver planchet against the 5.67-gram clad sister gives a calibrated scale a half-gram spread to resolve, the cleanest non-destructive separation. The reeded edge reads silver around the perimeter without the reddish copper-core line a clad strike shows at an angle. PCGS, the Professional Coin Grading Service, and NGC, the Numismatic Guaranty Company, label the slab insert with Silver or the S/Ag abbreviation alongside the proof designation, where Cameo (CAM) is moderate frosted-device contrast and Deep Cameo (DCAM) the higher tier that is the standard outcome on the issue. The 2018-S Block Island Silver Reverse Proof, distributed in the dedicated 2018 Silver Reverse Proof Set, is a separate catalog entry with mirrored devices against frosted fields (the inverse of the standard proof finish) and trades as an independent issue rather than a finish variant. Strike diagnostics on the two-bird composition sit at the night heron's wing feathers, the oystercatcher's distinctive bill, and the chick relief in the lower foreground.

The collector market positions the issue as the closing 90%-silver ATB release, and the closing-composition status gives Block Island a modest cross-batch pull beyond the typical Silver Proof Set assembly demand. PR69-DCAM examples in PCGS or NGC holders move at a modest premium over the silver-spot floor carried by the 0.1808 ounce content, and PR70-DCAM examples sit a tier higher with registry-set buyers driving the spread. The 451,510 silver print run is the lowest annual silver-set figure of the second half of the 90%-silver ATB run, and the two-species avian subject draws Rhode Island state-coinage and wildlife-design buyers as a secondary cohort. For the broader story of the ATB Silver Proof program and the series' production arc, see the Washington ATB series history.