Altar crystals are those that help to create a special and sacred space. These crystals need not be large, although they generally are, but do need to be very energetic, have a special significance for their Keeper and truly set the stage for the type of altar you want. Many have set up healing altars to deal with health issues and choose crystals like golden healers, elestials, growth interference quartz, warrior crystals, chlorites, etc. Heart altars add rose quartz, rhodochrosite, manganocalcite, citrine, Herkimer diamonds, etc. Meditation altars are the loftiest of all and should include all your etherial crystals that open up the Crown and causal Chakras. Yet others are representatives of crystals used for each Chakra - like amethyst, citrine, carnelian, etc. The choices are all up to you and only you can make the altar uniquely yours!
Don't be afraid to add shells, feathers, sage and other items that seem to add to the sacred feel!