Hidden within a shell so tight, A gem of ocean, pure and white. What am I?

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Across
  1. 2. Hospital visitor (7)
  2. 5. Brain teasers (7)
  3. 6. Retaliation (7)
Down
  1. 1. Feeling alone (10)
  2. 3. Safe haven (6)
  3. 4. Remedy (4)