Important moments seemed to sparkle.
More accurately, moments Lucy spent with Loke seemed to sparkle. But those moments were important and important moments sparkled.
At first, they sparkled from the second she summoned him with a shining golden key to the flash when he disappeared with a wave and a wink. The moments she and Loke had then were filled with flirtatious remarks and begrudgingly affectionate quips. Their partnership was new, teasing, and despite the very nature of the circumstances which resulted in Lucy calling on him, rather fun. Loke was not only a very accomplished spirit, but a good friend, and sometimes Lucy wished he would stay after their work was done. The world was always dimmer after he left; it was no longer as vivid and sparkling.
Later, everything sparkled in a new way. These moments twinkled like a star, a constant source of light and beauty. They gleamed and shone from when Lucy was dressing in a shimmering silver gown to when Loke was pouring wine into shining clear glasses. They sparkled through the heightened senses of the nervous and the hopeful and the infatuated. Lucy's partnership with Loke had changed and now he stuck around for a while after being summoned far more often than not. The moments that Loke called dates and Lucy tried to call meetings were pleasant and witty and amusing and slowly being tinted with rosy hues. Lucy looked forward to them more than she admitted and Loke always said they were the highlight of his existence.
After a while, the moments they spent together began to glow, like coals in a hearth. They were peaceful and contented and warm. Sometimes they sparkled again, like the first time Loke showed up outside Lucy's house using his own magic, and the first time Lucy pressed her lips to his. The tone of the sparkling and shining and glowing of their moments sometimes changed, turning pink when they openly kissed in a café and gleaming gold when Loke appeared to be with Lucy after a bad day and burning red when they fought, but the world was always in bright, harsh, clarity when they were together. Lucy wouldn't have it any other way.
Loke hung around in Earthland more often than was necessary, most of the time standing beside Lucy, but sometimes loitering around the guild. His presence was familiar and natural; no one was surprised to see him anymore and everything always seemed to be in bright sparkling hues. Lucy and Loke's relationship was the same as their moments were: long-lasting, vivid, and comfortable. Lucy would be happy to spend eternity in such moments. She never wanted the world to seem dim again.
Her wish was granted in a moment that sparkled like they had only just realized their feelings for each other. The stars twinkled, the hearth glowed, and Loke might have been shining with golden light as he pulled out and opened a box with a sparkling diamond ring in it.
Sometime later, the room literally sparkled with silver and gold as Lucy and Loke exchanged shining golden rings. The kiss that followed was enough to put the glimmering of Loke's proposal to shame. The moment sparkled with vibrant clarity.