Spark crystal
From Transformers Wiki
Introduced with the Transmetal 2 toyline in 1999, spark crystals are chromed insignia covered by a plastic bubble in various colors, acting as the replacement for the toyline's earlier Energon Chips. They saw use for about the next 3-4 years, and occasionally popped up later when toys would be reissued in later lines.
In the Beast Wars cartoon series, in at least Dinobot II and Cheetor, the placement of the spark crystal on the toy represents the actual placement of the Transformer's spark in the body (Dinobot II's beast mode head/robot mode chest; Cheetor's left thigh). The Transmetal 2 Blackarachnia toy had two spark crystals (one Predacon, one Maximal), as her upgrade to Transmetal 2 removed her Predacon shell program in the process.
In Beast Machines, the certainty of this is a bit fuzzier. The Maximals' sparks seem to be distributed throughout their bodies, only becoming the glowing orb when being extracted, so it could explain away some of the stranger places to put a spark crystal (like Geckobot's, which basically serves as their eyes and brain). And while this could also explain the presence of spark crystals on the Vehicon Generals, the placement of spark crystals on Vehicons meant to be mindless drones doesn't make much sense (particularly the "Battle for the Spark" during the latter half of the television series).
In Beast Wars Neo, spark crystals seem to serve no other purpose other than to identify Maximals from Predacons, much like the rubsigns of old.
In the 2001 Robots in Disguise toyline, spark crystals were only featured on the Predacon subgroup. In the show, it fired a sort of laser blast.
Design-wise, by around 2002-2003 the spark crystals were only being molded with the Beast Machines Dinobot symbol, the toolings for other symbols no longer in use for whatever reason. Subsequent releases for non-Dinobot toys either saw the "wrong" symbol used (i.e. BotCon 2016 TM3 Megatron), or used a tampograph to show the intended faction symbol (many Universe toys, BotCon 2005 Deathsaurus), or replaced the translucent plastic with an opaque dome showing no symbol at all (such as Robots in Disguise Storm Jet or BotCon 2010 Double Punch) (though technically the latter is of the second crystal design, mentioned below).
A different design of spark crystals were used in the Energon line; they were much smaller, meant to have Energon stars plugged on top of them. Presumably, the crystal's insignia is supposed to be magnified by the translucent center of the Energon star; in practice, this did not work all that well and the insignia was often very blurry when viewed through the star. Many promotional photographs showed colored spark crystals on these figures, in red for Autobots and green for Decepticons.
