Stop now, Briony: That sounds like jealousy, and you know what happens when you get jealous. Your witchy jealousy breeds firestorms, gales, floods—disasters of Biblical magnitude. Wouldn't father be proud! (4.58)
Briony envisions physical damage that can happen due to her magical powers mixing with jealousy. She references the Bible, linking the power of her jealousy to that of a jealous God. Though her fears of disasters of "Biblical magnitude" may be unwarranted, jealousy certainly can cause damage.
When you're jealous, your spit turns to acid. When you're jealous, you eat yourself from the inside out. (4.118)
Okay, Briony is playing a poet here. She discusses the internal damage caused by jealousy, using two metaphors to express the pain, suffering, and emotional destruction that can be caused by this emotion.
Jealousy makes you feel small as a splinter. Jealousy makes you feel empty, makes you want to reach for the Brownie. (5.93)
When Briony says jealousy makes "you feel empty," she is really saying that it makes her feel empty. By mentioning wanting to reach for Brownie in relation to jealousy, she leaves us wondering if she wants Brownie for comfort, to help her do damage through magic, or both.
The clever Briony knows that when she enters the swamp, people die. The clever Briony intended that Rose contract the swamp cough. She has always been jealous of Rose. This to her is the third strand of Hell. (9.103-104)
When Briony fails to save her sister (actually putting her in harm's way instead), she convinces herself that she did it on purpose out of jealousy. She seems convinced that this is true, but also describes it as "the third strand of Hell," leaving us to wonder why Briony would do something that causes her so much pain.
But she didn't breakfast with Eldric every morning, as I did. She didn't laugh with him as they expanded their bad boy Latin vocabulary. She didn't have boxing lessons with him, and surely, he never admired her fist. Did he? (14.69)
As she begins to have feelings toward Eldric (whether they are friendship or love feelings remains to be decided) Briony also begins to notice the attention he gives to Leanne. She tells herself things like this to lessen her feelings of jealousy.
There are no preconditions for jealousy. You don't have to be right, you don't have to be reasonable. (16.145)
When she notes that you don't have to be right or reasonable, Briony is admitting that her jealousy toward Leanne and Eldric may not be either. She goes on to compare the situation to Othello, suggesting that though she isn't right or reasonable, she wouldn't mind seeing Leanne dead.
I was jealous, wasn't I? I wanted to be Eldric's only friend. But that's not the way the world works, Briony. You have only one friend, but regular people have dozens.
Yes, I was jealous. I was practicing one of the seven deadly sins (although it doesn't actually take much practice). (16.94-95)
Briony admits her jealousy to herself, but frames it as a friendship-type jealousy. We're not convinced that's all that's at play here, but props to Briony for admitting at least part of the truth. When she says jealousy doesn't take much practice, she is admitting that she has felt this emotion easily and often.
I'd had visions of Eldric and Leanne on the hayride. Drinking from the same thermos; sharing a blanket; and when their fellow hay-riders left, lingering, perhaps, in the hay— (20.69)
The jealousy that Briony feels causes her thoughts to create and replay a painful vision of reality for her. By thinking of these things in detail, Briony's jealousy is essentially punishing her with heartbreak.
Cecil's lips were wet. "By God, you'll kiss me too!" Kiss me too! Fear whispered at the margins of my thoughts. (21.21)
Cecil has been jealous of Eldric since he first saw Briony and Eldric in the same room together—and he has never tried to conceal or control his jealousy, and this night is no different. What is different, however, is that he combines his jealousy with lust to make a very dangerous situation for Briony.
"Leanne, worried about you?"
Eldric punched at the you as though he were boxing with it. The punch came like a kick to the breastbone. I shrugged as though to say, Believe what you like. But I had no breath to speak. (23.84-85)
When Briony suggests that Leanne could have tried to harm Briony out of jealousy, Eldric's response hurts Briony. By not even considering Briony a threat to Leanne, Eldric is basically saying he doesn't see Briony that way, which stings like hearing let's just be friends from your crush.