What you need:
One glass of wine
Wine(pref not expired)
Three Candles ( pref red but it's ok you can use any candle)
Incense ( pref Storax )
Take the rocks from a place with dirt , take them and clean them with water then put them next to fire for 30 Minutes and 30 minutes outside . Then take them inside and put them into your alter or somewhere else. Lit the three candles and put the amethyst on the rocks then fill the glass with wine and say the following:
Bacchus [Dionysos] I call, loud-sounding and divine, fanatic God, a two-fold shape is thine:
Thy various names and attributes I sing, O, first-born, thrice begotten, Bacchic king:
Rural, ineffable, two-form'd, obscure, two-horn'd, with ivy crown'd, euion, pure.
Bull-fac'd, and martial, bearer of the vine, endu'd with counsel prudent [Eubouleos] and divine:
Triennial, whom the leaves of vines adorn, of Jove [Zeus] and Proserpine [Persephoneia] , occultly born.
Immortal dæmon, hear my suppliant voice, give me in blameless plenty to rejoice;
And listen gracious to my mystic pray'r, surrounded with thy choir of nurses fair.
I ask you , Protect this area(or house) from evil and negativity and guard us oh Immortal dæmon.
Then visualize from these two rocks a shield that protects your desired area . That's the spell :)