There have been three types of plastic Icehouse pieces available for commercial sale.
The first were cast in polyester resin. These were sold by Icehouse Games, Inc., mostly in the First 100 box sets. These are more commonly referred to as the "resin pieces". These pieces were solid, and appeared beautifully crystalline. Unfortunately, they were quite brittle and subject to having their tips broken off.
The second type of plastic pieces were cast in high-impact plastic. They were sold in the later days of Icehouse Games. While, not as brittle as the resin pieces, they were subject to many irregularities and were never produced in great quantity. These pieces are commonly referred to as the "unbreakable pieces" due to their enhanced durability.
The third, and most common type of plastic piece is the injection-molded styrene pieces (crystal polystyrene DOW 666D) sold by Looney Labs. These pieces are hollow, stackable, and have pips molded into the sides to make telling the sizes apart easier. These have been manufactured in large numbers, in at least twelve different colors. They have been the core of three different boxed sets (Icehouse: The Martian Chess Set, Zendo, and IceTowers) as well as being sold as monochrome stashes in square tubes. Since the 2010 relaunch of the Pyramids they have been sold in the IceDice pyramid bag, as well as tuckboxes of Rainbow and Xeno stashes.
This third type of plastic piece is what most people refer to as "plastic pieces", although they are sometimes called "hollow pieces", "stackable pieces", or "modern pieces". Their stackability makes the Icehouse game system more versatile, and their availability has made Icehouse games more popular than ever.
- Here's a video of Kristin Looney and Alison Frane sorting modern stacking pyramids for inclusion in limited edition Treehouse sets.