Transparency, translucency and surface reflectance are material qualities that underpin modernist aesthetics. The lightness and transparency of buildings remain manifestly contemporary and the walls of our cities continue to open up, as the default material of choice is glass. From London to Los Angeles, from the Shard to the Crystal Mall, the potency and immateriality of the crystal image is a liberation of the modern imagination. This paper concerns the crystal imagination as represented in the architectural interiors and poetry of Belgian symbolism. During the last decade of the nineteenth century, aesthetic theory and output in Brussels was, remarkably, more developed than that of other European capitals. For the Symbolist artists, coloured and patterned glass, mirrors and rare stones were deeply fascinating as they were at once transparent (see-through) and also contained an interior mysteriously closed to the world outside. As a reflection on the dominance of the visible in contemporary architecture, this paper explores the dual aspects of the crystal image – as both open and closed - and the notion of interior as it matures in the work of the architect Victor Horta. Here, a highly subjective taste for the artificial is combined with themes that surround the crystal and which continue into the materialism of modernist movement through German expressionism. Bruno Taut's Cologne Werkbund Exhibition Glashaus (1914) embodied Scheerbart's crystal visions, and went on to inspire the activist movement that was to deliver such dreams into political action. The paper comments on the development of Taut's ideas in modernism, and the correlation of glass and crystal to the singular experience of vision. It suggests that the crystal imagination of the late nineteenth century, and the potential for those crystal metaphors to disclose worlds not available to sight, still may provide rich inspiration for tomorrow's architectural visions.