Crystals of insoluble compounds
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Complex formation
Certain complexes, like Sodium dicarbonatocuprate(II), are insoluble in water but precitipates not very rapidly, when formed by mixing copper salts with Sodium hydrocarbonate. Then, small but still macroscopic crystals will form.
Melting
Certain materials, like metallic bismuth, may be crystallised via melting and slowly cooling the melt. Salt melts also form crystals when cooling slowly for few days: materials like lithium carbonate or certain oxides, commonly used as flux, has been known to crystallise this way.
Propagation of reactants
Use silica gel impreganated with each of the reactants on each side, crystals will form at boundary. As the concentration of reactants at boundary is small initially, growth will start slow enough to form singke crystals. Synthetic calcite single crystals of few milimeter size has been grown this way. Certain reports state that copper metal single crystal is grown on iron wire, when placed into a column of table salt soaked with copper(II) solution with some clean table salt in between as buffer; this can be a "poor man's method" to grow any insoluble compounds, e.g. synthetic monocrystalline gypsum can be prepared via propagation of CaCl2 and NaCO3 in salt/sand column.
Sublimation
Solids like iodine and many organic compounds sublimate and such vapor form sizable crystals on cold surfaces.
Gallery
Calcium carbonate crystals
Bismuth hopper crystals