The Letter 'O'
Today is Tuesday, 17th April and the letter is 'O'. 'O is for Orange' and O is for Opals'.
By the way, according to Victoria Finlay's book, Jewels, the Opal scores 5 to 6.5 on Mohs relative hardness scale, harder than Amber, Jet and Peridot and Pearl, but softer than Emerald, Sapphire, Ruby and Diamond.
Duck and Wheel With String
And Lin is right. O is for Orange Stripey Goodness, an Orange Cat like Hobbes of the Red Cat Society