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The federal government has the ability to tax "income." Unrealized gains are not income as gains have not been clearly realized.
The closest legal definition for "income" comes from:
The Glenshaw Glass case
In Commissioner v. Glenshaw Glass Co., 348 U.S. 426 (1955), the Supreme Court laid out what has become the modern understanding of what constitutes "gross income" to which the Sixteenth Amendment applies, declaring that income taxes could be levied on "accessions to wealth, clearly realized, and over which the taxpayers have complete dominion". Under this definition, any increase in wealth—whether through wages, benefits, bonuses, sale of stock or other property at a profit, bets won, lucky finds, awards of punitive damages in a lawsuit, qui tam actions—are all within the definition of income, unless the Congress makes a specific exemption, as it has for items such as life insurance proceeds received by reason of the death of the insured party, gifts, bequests, devises and inheritances, and certain scholarships.
https://en.m.wikipedia.org/wiki/Sixteenth_Amendment_to_the_U...
See case law section