Good answer - thanks. Didn't think of item damage from the sphere.
Hardness of a clay pot designed to not break when thrown? I wouldn't want to call it stone (hardness 8), so perhaps something more like wood is the appropriate comparison. So Hardness 5, and for a 1/2" thick at 10 hp / inch, 5 hit points? These are key assumptions.
3d6 fire damage gives an average roll of 10.5, half damage for elemental damage vs. object, gives about 5 points of damage for the average roll, which is negated by the hardness. So the average roll would do no damage to the fuse grenade.
To put it another way, with hardness 5 and 5 hp, the fire damage roll would have to be 20 (20/2 = 10, 10 - 5 hardness = 5 hp damage), which is impossible with a single 3d6 roll. If you gave the pot had 4 hp it would be possible on a roll of 18 (1 in 216 chance), or 3 hp would make it possible on a roll of 16 (10 in 216). If you lowered the hardness of the clay pot to 4, then it would be possible to destroy a 5 hp pot on an 18, a 4 hp pot on a 16. Still pretty low odds.
"Go with a ruling that is unlikely to cause your players to be upset or be able to be abused by them in the future."
Solid advice. My players have a strong bent for power gaming and are on the hotheaded side. Sometimes they really surprise me on what they get angry about.