''The Stone'' Age

If the creators of Myst, The X-Files, and the Pet Rock joined forces for an Internet scavenger hunt, it just might look like The Stone. For $19.95, you get a rock on a rope that grants you access to a series of cryptic online puzzles. Go to www.thestone.net, enter the symbols carved into your stone, and take a crack at any of the mind teasers alone — or better yet, find the ''Stonemate'' whose symbols match your own. Only 25 of 212 total games have been posted to date; manufacturer Abject Modernity will add new puzzles biweekly until the end of next year. ''Each one represents a finite piece of the overall mystery,'' says game creator Rod Bruinooge. With 50,000 sold, this rock is on a roll.