StevenBomb 4 ended last month (don't know why I'm writing this one month later), and with its conclusion we got a lot more info about Homeworld and the Diamonds (the elite gems of Homeworld as described by Peridot in the StevenBomb 4 episode "It Could've Been Great") . Especially about how to become a Crystal Gem.
We learned that there are different ways to betray a Diamond and become a Crystal Gem. One reason is to disobey a Diamond's orders (as seen by Peridot in the other StevenBomb 4 episode "Message Received") and to fuse with two different gems (the first episode of StevenBomb 4 "The Answer" explains all that). So enough discussion, let's talk about Jasper and her being a Crystal Gem!
At the end of the episode "Jailbreak" (you now, the episode where Peridot and Jasper invade Earth, we find out Garnet is a fusion, and where Garnet sang the best Steven Universe song of all time) Jasper fuses with Lapis so they could take out the Crystal Gems, but Lapis' power is much more stronger than Jasper's, and uses it to bring her and Jasper's fusion (known as Malachite) into the bottom of the ocean. So how is Jasper a Crystal Gem?
It's simple. In the episode "The Answer" we know that a fusion of two different gems results in a gem betraying a Diamond and being kicked out of Homeworld!
Wait 'til Yellow Diamond hears about this!