The "Download Now" link directs you to the iTunes App Store, where you must continue the download process. You must have iTunes installed in order to open the link, and you must have an active iTunes account to download the application. This download may not be available in some countries.
From Matthias Hofmann:
Stone was designed to help musicians to find chords, substitutes and alternative voicings for a given set of harmonies and scales. Stone allows to define an arbitrary set of notes based on the chromatic scale - c, c#, d, d#, e, f, f#, g, g#, a, Bb, B. The set of notes is defined by adding chords, scales, single notes or by tapping on one of the following instruments and tunings:1. Guitar Standard Tuning2. Piano and Keyboard3. Bass Guitar Standard Tuning4. Mandolin Standard Tuning5. Bouzouki Standard Tuning6. Guitar Drop D Tuning7. Guitar Double Drop D TuningBased on set of notes Stone determines all chords and scales which ..1. equal set of notes.2. completely include set of notes and have additional notes.3. are completely part of but have less notes than defined in set. Additional notes are explicitly marked per chord and scale.Since all chord representations are displayed on a common set of instruments Stone is the application to find alternative voicings and substitutes and to learn about harmonies and their relations as well.