Taken from IRS Form 2004-50: HSA funds may be invested in investments approved for IRAs (e.g., bank accounts, annuities, certificates of deposit, stocks, mutual funds, or bonds). HSAs may not invest in life insurance contracts, or in collectibles (e.g., any work of art, antique, metal, gem, stamp, coin, alcoholic beverage, or other tangible personal property specified in IRS guidance under section 408(m)). HSAs may, however, invest in certain types of bullion or coins, as described in section 408(m)(3). The HSA trust or custodial agreement may restrict investments to certain types of permissible investments (e.g., particular investment funds).
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Source: IRS Notice 2004-50 Q&A Q-65