Ice Shards is the 3rd upgrade on Path 2 for the Ice Tower.
Bloons that are frozen from this tower have sharp ice shards embedded in the bloon layer, so that when a bloon is popped or thawed, these shards from the bloon fly out, popping additional bloons, including frozen ones (except the nerf in BTDB).
On higher rounds, this tower can be lethal, setting off huge bursts of shards which can clean out hundreds, even thousands of bloons! However, this tower comes at the price of possibly causing extreme lag to the player's device. In BTD5Sandbox, one player used regrow farming to make lots of bloons, placed down the Ice tower, upgraded it to 2/3, and in one freeze it popped over 100,000 bloons.
This upgrade is very useful for rounds with huge rushes of bloons like round 63, 76, 78 and 79, and especially regen rushes.
The tower seems to hold up its fists when this upgrade is purchased.
Despite the shards not popping lead, when a Lead Bloon is affected by a 2/3 Ice Monkey, the Lead Bloon can sometimes be completely popped. Has been tested in BMC Mobile on a Snow Terrain.
This upgrade also slightly increase freeze time, to the point that a 1/3 Ice Tower can permanently freeze bloons in place, despite this not being mentioned on the upgrade description. This can be seen by putting a 1/3 Ice Tower and comparing it to a 1/2 Ice Tower.