The word grindstone is playable in Words With Friends™, no blanks required. Because it is longer than 7 letters, you would have to play off an existing word or do it in several moves.
Words With Friends™ Letter Score: 15
Highest Scoring Words With Friends™ Plays In The Letters
Seven Letter Word Alert: (55 words)
denting, dentins, dinners, droning, editors, endings, eroding, godsent, goiters, goitres, goriest, grinned, groined, ignored, ignores, indents, indorse, intends, interns, intoned, intoner, intones, introns, negroid, nesting, nondiet, norites, orients, redoing, regions, rending, renting, resting, rodents, roisted, rosined, sending, snoring, snorted, sortied, sorting, steroid, stinger, stonier, stoning, storied, storing, tending, tendons, tensing, tension, tierods, tinders, tinners, triones