Spark crystal
Introduced with the Transmetal 2 in 1999, spark crystals are plastic bubbles in various colors reflecting a vacuum-metal insignia of the allegiance of whatever Transformer possesses it (Maximal for Cheetor, Predacon for Dinobot 2, etc.), and used in pretty much every Transformers continuity since.
In the Beast Wars cartoon series, in at least Dinobot 2 and Cheetor, the placement of the spark crystal on the toy represents the actual placement of the Transformer's Spark in the body (Dinobot 2'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 TM2 was apparently the moment she began considering herself a Maximal (e.g., she began using "Maximize" as her transformation code).
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 his 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 TV series.
In Beast Wars II and 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 Robots in Disguise line, spark crystals were only added to the Predacon sub-group. In the show, it fired a sort of laser blast.
With the rise of the Energon line, spark crystals shrank, until they became a tiny gray insignia in the hole where you plugged in the Energon star. Presumably, it is supposed to be magnified by the translucent center of the Energon star.
