Hi Brion,
For washing and cleansing rituals, then enact such rites under a new Moon at the very least as far as natural conditions are concerned, so as the Moon waxes, your gems will be imbued with new life after their cleansing. I could elaborate a lot upon the importance of natural conditions.
A few ideas.......
Simply take each gem and ritually hold it under running water, or better still wash them in a stream or river to add greater momentum to the act of cleansing. It would be good to write down some sort of incantation to learn and recite as you do this, to give the ritual more power of intent.
It's not a bad idea to take these gems into the bath with you, cleansing yourself and the stones at the same time, washing away your old energies along with the stone's old vibes. This would be a very intimate way of acquainting your energies with the gem's and the gems with your's. Make this bath a special, ritual occasion. Adding sea salt, a potent psychic cleanser, to the water would increase the potency, think also of essential oils or other ingredients you could add which would signify the forces of cleansing, purity and renewal.
Another idea is to 'smudge' your stones, by holding over them over the smoke of incense, or wafting the smoke over them in order to ritually cleanse them. Again, think about this ritually, use incantation or otherwise elaborate in order to focus intention and direct the will. Use an incense which would embody the energies of purging and regeneration. I would suggest Dragon's Blood, but whatever feels most appropriate to you.
Another idea, which is more meditational, is to take your passive or receptive hand (usually the left), which draws in energy, and hold this hand, palm open, over the stones, or each stone, and then visualise the unwanted energies which have accumulated in the gems passing up through your left arm and across your chest into your heart. Then imagine this energy being transported and purified through your heart centre, before being passed out of your active hand, which projects energy, into the earth or into a tree where the energies may be given back to nature, which will cause no harm, because the energies you would wish to dispel would have been purified in your heart centre.
You will get out of such rituals what you put into them!
For a particularly powerful cleansing of your gems, then you could do all of these things consecutively. I very much doubt there would be much trace of the previous owner left!
Anyway, just my ideas.
X