The LaTeX command for diamondsuit (♦) is \diamondsuit.
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Write \diamondsuit inside math mode, that is between $…$ for inline math or in a \[ … \] or equation block for display math. It is part of core LaTeX math, so no extra package is required.
\diamondsuit is a math-mode symbol: in running text, wrap it in $...$. With unicode-math under XeLaTeX or LuaLaTeX, typing ♦ in a formula gives the same symbol.
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Use \diamondsuit in math mode; it needs no extra package. For example, $\diamondsuit$.
None: \diamondsuit is built into LaTeX, so no extra package is required.