Stone Tool is one of the three skins released as part of the Stone Age coin deal. It could be purchased as part of an item deal for a limited time in mid May 2017.
Stone Tool could be purchased for $9.99 in a deal that included 16000 coins. It could also be purchased for $19.99 in the "Best Deal!" which included the Fire Face skin and either 50000 coins (Browser) or 1800 DNA (Mobile). Stone Tool was not available for individual purchase and may not be available for purchase in the future.
Details of this skin may vary depending on location, platform or time.
Note: The Stone Age offer with the Stone Tool skin was released again for a limited time in early March 2019. This time the deal that included 16000 coins was lowered to $4.99. The Best Deal was lowered to $9.99 and only included 30000 coins. The package of DNA was no longer included as an option.