I work with the goddess of the moon, known as Luna and i was wondering, can you place crystals on a alter as a offering, sense i don't want to have things like food and water out in the open, and have like bugs crawl over the food/water.
You can place crystals as an offering. If you are worried about bugs falling into the water or in the food then you can remove the food and water by the end of the day. With food offerings for me I normally leave them out in the day and let them stay for the night and remove them in the morning, but crystals work just as good.
Yes you can place and leave crystals on the altar as offerings.
You can also use fresh or dried flowers in place of food as well.
Incense as well can be used as an offering.
There are many choices other than that of food and water that you have available for use as offerings.
Some offer shells, money, tobacco, alcohol, etc depending on dieties they work with and preferences.
It depends on your preference and practice what you choose to offer.
It's your choice and it need not be consumables at all if you prefer it not be.
There are some practitioners whom will dedicate certain colors to their deities and use items of that or those colors to offer. I've done this in the past.
I don't personally offer food often, and never water, though I have in the past offered alcohol for a time, bottled and capped, until ready to share in a celebratory toast to Deity.
Currently my offering consists of a dried arrangement of wildflowers in a vase, which serves as a longer term offering until I am ready to add or exchange it for something else.
I am sure she will be glad with crystals. I know some people cant leave offerings of food or water for family or pestreasons and I bet she will understand. Try looking for other offerings you could give her like flowers, incense, poetry, or even rituals honoring her.
You don't need candles, same with spells while many people may prefer them they are not necessarily. Like the name suggest in witchcraft. It's all about the craft aspect, be creative, if you are good at drawing or writing you could also offer that as an offering aswill.