An alternative asset is any investment asset not publicly traded. The IRS Code governing what types of investments can be held within a non-taxable retirement account does not specify what can be held within the account but rather what cannot be held. This means that your investment options are nearly infinite.
Some of the more common investments held in self-directed accounts are:
Bitcoin and other digital assets
Real estate (commercial and residential property, tax liens, rental property, etc.)
Precious metals (such as gold and silver bullion, American Eagle coins and platinum bars)
Private companies (including LLCs, limited partnerships and C corporations)
Private lending (such as notes secured by deeds of trust, unsecured notes and mortgages)
Single-member LLCs
Private REITS and funds
Crowdfunding and marketplace lending opportunities
Some less common investments held in a self-directed account include:
Timber rights
Cattle
Equipment leases
Virtually anything else!