The short answer is that any asset besides cash should be considered an intermediate to long-term investment, since if you needed to sell in a short time frame, you might get less than your original investment. So exclude your cash reserves for short term needs from any longer-term investments.
But beyond knowing that we wouldn’t use precious metals for cash needs, whether precious metals are a long-term or short-term investment depends on your motive for holding them.
If your motive is portfolio diversification, or to hold something tangible in times of financial distress, then metals can play a specialized role in your portfolio. For the longer term investor, a small allocation to metals can diversify a portfolio with both traditional and alternative assets. For example, the diversifying effect of gold (along with many other statistics) is available from the World Gold Council here.
Traders who attempt to make money on short-term price swings like precious metals for their volatility, especially with the introduction of exchange-traded funds based on precious metals that eliminate the difficulty of owning physical metal. Interestingly, although gold is thought of as highly volatile, it is no more so than many equity asset classes, according to the World Gold Council. Of course, equities are another asset class used by long-term and short-term investors alike for different reasons.