Only cash contributions (e.g., checks, credit card charges) are acceptable in 529 plans. You’ll have to sell the stock and donate the proceeds. If the stock has appreciated, you’ll owe capital gains tax on your profits; if the value has declined, you’ll be able to take a tax loss.
Depreciable property used in a trade or business and held for more than a year. All Section 1231 gains and losses are netted; a net gain is treated as capital gain, a net loss as an ordinary loss.