diff --git a/backend/card.py b/backend/card.py
index c1bbc2c..93af87d 100644
--- a/backend/card.py
+++ b/backend/card.py
@@ -243,6 +243,7 @@ WIKIDATA_INSTANCE_TYPE_MAP = {
   "Q24034552":  CardType.science_thing, # mathematical concept
   "Q12089225":  CardType.science_thing, # mineral species
   "Q55640599":  CardType.science_thing, # group of chemical entities
+  "Q17339814":  CardType.science_thing, # group or class of chemical substances
   "Q119459661": CardType.science_thing, # scientific activity
   "Q113145171": CardType.science_thing, # type of chemical entity
 
diff --git a/backend/main.py b/backend/main.py
index 6b0da49..36f8a1a 100644
--- a/backend/main.py
+++ b/backend/main.py
@@ -163,6 +163,14 @@ def get_cards(user: UserModel = Depends(get_current_user), db: Session = Depends
     for card in cards
   ]
 
+@app.get("/cards/in-decks")
+def get_cards_in_decks(user: UserModel = Depends(get_current_user), db: Session = Depends(get_db)):
+  deck_ids = [d.id for d in db.query(DeckModel).filter(DeckModel.user_id == user.id, DeckModel.deleted == False).all()]
+  if not deck_ids:
+    return []
+  card_ids = db.query(DeckCardModel.card_id).filter(DeckCardModel.deck_id.in_(deck_ids)).distinct().all()
+  return [str(row.card_id) for row in card_ids]
+
 @app.post("/open_pack")
 @limiter.limit("10/minute")
 async def open_pack(request: Request, user: UserModel = Depends(get_current_user), db: Session = Depends(get_db)):
diff --git a/frontend/src/lib/CardSelector.svelte b/frontend/src/lib/CardSelector.svelte
index e5cce3c..9dabd14 100644
--- a/frontend/src/lib/CardSelector.svelte
+++ b/frontend/src/lib/CardSelector.svelte
@@ -8,6 +8,7 @@
   let {
     allCards = [],
     selectedIds = $bindable(new Set()),
+    inDeckIds = new Set(),
     onclose = null,
     costLimit = null,   // if set, prevents selecting cards that would exceed it
     showFooter = true,  // set false to hide the Done button (e.g. inline deck builder)
@@ -185,6 +186,9 @@
             {#if selectedIds.has(card.id)}
               <div class="selected-badge">✓</div>
             {/if}
+            {#if inDeckIds.has(card.id)}
+              <div class="in-deck-badge" title="In a deck">⊞</div>
+            {/if}
           </button>
         {/each}
       </div>
@@ -410,6 +414,21 @@
     z-index: 10;
   }
 
+  .in-deck-badge {
+    position: absolute;
+    top: 6px;
+    right: 6px;
+    background: rgba(13, 8, 2, 0.75);
+    color: #7ecfcf;
+    font-size: 16px;
+    line-height: 1;
+    padding: 3px 5px;
+    border-radius: 6px;
+    border: 1px solid rgba(126, 207, 207, 0.5);
+    pointer-events: none;
+    z-index: 10;
+  }
+
   .status {
     font-family: 'Crimson Text', serif;
     font-size: 16px;
diff --git a/frontend/src/routes/shards/+page.svelte b/frontend/src/routes/shards/+page.svelte
index 20c2898..6b59c48 100644
--- a/frontend/src/routes/shards/+page.svelte
+++ b/frontend/src/routes/shards/+page.svelte
@@ -11,20 +11,23 @@
   let selectorOpen = $state(false);
   let shattering = $state(false);
   let result = $state(null); // { gained, shards }
+  let inDeckIds = $state(new Set());
 
   const selectedCards = $derived(allCards.filter(c => selectedIds.has(c.id)));
   const totalYield = $derived(selectedCards.reduce((sum, c) => sum + c.cost, 0));
 
   onMount(async () => {
     if (!localStorage.getItem('token')) { goto('/auth'); return; }
-    const [cardsRes, profileRes] = await Promise.all([
+    const [cardsRes, profileRes, inDecksRes] = await Promise.all([
       apiFetch(`${API_URL}/cards`),
       apiFetch(`${API_URL}/profile`),
+      apiFetch(`${API_URL}/cards/in-decks`),
     ]);
     if (cardsRes.status === 401) { goto('/auth'); return; }
     allCards = await cardsRes.json();
     const profile = await profileRes.json();
     shards = profile.shards;
+    if (inDecksRes.ok) inDeckIds = new Set(await inDecksRes.json());
   });
 
   async function shatter() {
@@ -95,6 +98,7 @@
     <CardSelector
       allCards={allCards}
       bind:selectedIds={selectedIds}
+      {inDeckIds}
       onclose={() => { selectorOpen = false; }}
     />
   </div>